What cookies are and what they do
A cookie is a small text file that a website stores on your device. When you visit that website again, your browser sends the cookie back to it automatically. The website reads what's in the cookie and uses that information to remember things about you — your login status, items in your shopping cart, your language preference, or pages you've visited.
Cookies exist because the internet was built without memory. Each time your browser requests a page, the server has no way to know if you've been there before or who you are. Cookies bridge that gap. Without them, you'd have to log in to your email every single time you refreshed the page, and online stores couldn't keep track of what you wanted to buy.
The cookie itself is just text — typically a few hundred bytes. It might contain your username, a session ID (a temporary token that proves you're logged in), or a tracking number. The website decides what goes in it and how long it stays on your device.
Key Takeaways
- Cookies are small text files websites store on your device so they can remember information about you between visits.
- First-party cookies come from the website you're visiting; third-party cookies come from advertisers or analytics companies and track you across multiple sites.
- You can see what cookies are stored, delete them, or block new ones from being created in your browser settings.
- Cookies expire on a date set by the website — some last only while your browser is open, others stay for months or years.
- Blocking all cookies may break some website features, but blocking third-party cookies usually has no effect on the site itself.
First-party cookies versus third-party cookies
A first-party cookie comes from the website you're actually visiting. When you log into Gmail, Google sets a cookie on your device. When you add something to your cart on Amazon, Amazon sets a cookie. These cookies let the website function — they're why you stay logged in and why your cart remembers what you put in it.
A third-party cookie comes from a different company than the one whose website you're on. An advertising network like Google Ads or Facebook Pixel might set a cookie on your device while you're reading a news article. That cookie tracks which sites you visit, what you click on, and what you search for. The ad network uses this information to build a profile of your interests and show you targeted ads across the internet.
Third-party cookies are why you see ads for something you looked at on one website while you're browsing a completely different site. They follow you across the web. First-party cookies stay within the website that set them — Amazon doesn't see the cookies Gmail set on your device.
Most modern browsers now block third-party cookies by default. Safari, Firefox, and Chrome (starting in 2024) all restrict them. You can also block them manually in your browser settings. Blocking third-party cookies usually doesn't break websites; it just stops advertisers from tracking you across sites.
How cookies get created and stored
When you visit a website, the server sends back a response that includes an instruction to set a cookie. Your browser reads that instruction and stores the cookie in a folder on your device. The location depends on your operating system — on Windows it's usually in AppData, on Mac it's in Library, on Linux it's in a hidden folder in your home directory.
You can see your cookies by opening your browser settings. In Chrome, go to Settings > Privacy and Security > Cookies and Other Site Data. In Firefox, go to Settings > Privacy & Security > Cookies and Site Data. In Safari, go to Preferences > Privacy > Manage Website Data. You'll see a list of every website that has stored a cookie on your device, how many cookies each one has, and how much space they take up.
Each cookie has an expiration date. Some cookies expire when you close your browser — these are called session cookies and they're temporary. Others have a specific date set by the website, sometimes months or years in the future. These are called persistent cookies. A website might set a persistent cookie that lasts a year so it can remember you the next time you visit, even if you close your browser in the meantime.
What information cookies actually contain
A cookie is just text, so it can only hold information that's been converted to text. It can't contain your passwords, your credit card number, or your full browsing history — those would be too large and too sensitive. Instead, cookies typically hold a session ID, which is a random string of characters that the website uses as a key to look up information about you on their server.
When you log into a website, the server creates a session ID and stores it in a cookie on your device. Every time you request a page, your browser sends that session ID back to the server. The server looks up that ID in its database and knows who you are and what permissions you have. If someone steals your session ID, they can impersonate you until the session expires — which is why websites use HTTPS (encrypted connections) to protect cookies in transit.
Some cookies do contain readable information — your language preference, your username, or a tracking number. But even these are limited in size. A cookie can be at most 4 kilobytes, which is about 4,000 characters of text. That's enough for a username and some settings, but not much more.
Why websites use cookies instead of other methods
Cookies are the oldest and simplest way for websites to remember you. They've been part of the web since 1994. The alternatives exist — websites can use local storage (a larger storage space on your device), session storage (temporary storage that clears when you close your browser), or server-side sessions (storing everything on their server) — but cookies are still the standard because they work everywhere and they're built into every browser.
Local storage can hold much more information than cookies — up to 5 or 10 megabytes depending on the browser — but it's only accessible to JavaScript code running on the page. Cookies are accessible to both the browser and the server, which makes them more flexible. A server can set a cookie without any JavaScript involved, and the browser will send it back automatically on every request.
Cookies are also the only method that works across different domains. If you log into your Google account, Google can set a cookie that your browser will send back to Google on every visit. But that cookie won't be sent to Facebook or Amazon — each site only receives its own cookies. This isolation is a security feature. It prevents one website from reading another website's cookies.
How to manage cookies in your browser
You can delete cookies at any time. In Chrome, go to Settings > Privacy and Security > Clear Browsing Data, check "Cookies and Other Site Data", and click Clear Data. You can choose to clear all cookies or just the ones from a specific time period. In Firefox, go to Settings > Privacy & Security > Cookies and Site Data and click "Clear Data". In Safari, go to Preferences > Privacy > Manage Website Data, select the sites you want to remove, and click Remove.
You can also block cookies from being created in the first place. Most browsers let you choose between three levels: allow all cookies, block third-party cookies, or block all cookies. Blocking all cookies will break many websites — you won't be able to stay logged in, your shopping cart won't work, and some pages won't load properly. Blocking third-party cookies is usually safe; it stops advertisers from tracking you but doesn't affect the website's core functions.
Some websites ask for your permission before setting cookies. This is required by law in many countries, including the European Union. You'll see a banner asking if you consent to cookies. If you click "Reject", the website should not set non-essential cookies. If you click "Accept All", it will set everything. Some sites also offer a "Manage Preferences" option where you can choose which types of cookies to allow.
The privacy and security trade-offs of cookies
Cookies make the web more convenient — you stay logged in, websites remember your preferences, and you don't have to enter the same information repeatedly. But they also let websites and advertisers track your behavior. Third-party cookies create a detailed profile of your interests, which advertisers use to target you with ads and which data brokers buy and sell.
Cookies themselves aren't inherently insecure. A website can't read cookies set by another website, and cookies are sent only to the domain that set them. But if a website is compromised or if someone steals your device, they can access your cookies. Session IDs in cookies can be stolen if you use an unencrypted connection (HTTP instead of HTTPS), which is why find websites always use HTTPS.
The real privacy concern is tracking. Advertisers use third-party cookies to follow you across the web and build a profile of what you like, what you search for, and what you buy. This information is valuable and is often sold to other companies. Blocking third-party cookies reduces this tracking significantly, though it doesn't eliminate it — websites can still track you using other methods like fingerprinting (analyzing your browser and device characteristics) or login-based tracking (following you because you're signed into their service).
Frequently Asked Questions
Can websites see my passwords or credit card numbers in cookies?
No. Cookies are limited to 4 kilobytes of text, and websites don't store sensitive information like passwords or credit card numbers in them. Websites use cookies to store session IDs, which are temporary tokens that prove you're logged in. The actual sensitive information stays on the website's server, protected by encryption and access controls.
What's the difference between clearing cookies and blocking them?
Clearing cookies deletes the ones already on your device. Blocking cookies prevents new ones from being created. If you clear your cookies but don't block them, websites will set new cookies the next time you visit. If you block cookies but don't clear them, the old ones stay on your device until they expire or you manually delete them.
Will blocking cookies break websites I use?
Blocking all cookies will break most websites — you'll be logged out, shopping carts won't work, and some pages won't load. Blocking only third-party cookies usually has no effect on the website itself; it just stops advertisers from tracking you. Most browsers now block third-party cookies by default, and websites still function normally.
Do I need to delete my cookies regularly?
Not necessarily. Cookies take up very little space — even thousands of them use only a few megabytes. Deleting them won't speed up your browser or free up meaningful storage. The main reason to delete cookies is privacy: clearing them removes tracking cookies and logs you out of websites. If you're concerned about tracking, blocking third-party cookies is more effective than periodically deleting them.
Can cookies give me a virus or malware?
No. Cookies are just text files. They can't execute code or install software. A malicious website could set a cookie that tracks you or stores information about you, but the cookie itself can't harm your device. The danger comes from the website you're visiting, not from the cookie.