CogBlog

Lately I've heard and read a lot of mis-information about cookies. And, by cookies I mean http cookies not the warm little dollops of baked sugar-dough (although I'm sure there is plenty of mis-information about those too).

Cookies are little bits of information that are stored on your computer by your web browser. They are usually used to identify you or your preferences (such as your shopping cart id) when you visit a website. A website can place a cookie on your machine, but you have control over this process.

There are some important restrictions on cookies, probably the biggest of which is that only the website that places the cookie, can read the cookie contents. So, if you go to Nike.com and they place a cookie while buying a new pair of shoes, the next site you visit will have no way to see this cookie. This is important because it dispells the most common myth: That sites use cookies to spy on you as you travel around the web. They don't. Not because they don't want to, it's because they can't tell where you go when you are not on their site.

This is also where another bit of controversy pops up, the third party cookie. A third-party cookie describes a cookie placed by a site other than the one who's URL appears in your web browser's address bar. Usually this is from an advertising or tracking service of some kind. For instance, say I'm on spoof news site The Onion and they are displaying a banner from an ad network. When the Onion page loads, my browser also requests the banner from the ad network, so really i'm browsing two sites at once. When I make the banner request, they can place a cookie from the Ad Network site as well. But, the ad network can't see the Onion's cookies, and vice versa.

So conceivably, a widely distributed ad network could track your movements across multiple sites--anywhere their ads appeared. This was the source of a big news story a few years ago involving DoubleClick. However most browsers now allow you to block all third party cookies. And, quite a number of people do just that, and blocking of third party cookies is the default setting on current versions of Internet Explorer.

Nowadays, most ad networks and tracking services use first-party cookies, meaning a cookie associated directly with the site you are viewing. This is innocuous because even if the same person was tracked on multiple sites, there would be no means of knowing which of those users were the same person, and which were unique individuals--no way to track from site to site.