From Ad Hacker Wiki

Website owners and advertisers are *very* interested in how you use browse the web and what you look at. While many have your best interests in mind, some are purely business people who want to know every detail about you, and stick whatever advertisement in front of you that will make them the most money. Some don't even put up ads, but instead sell your information to other advertisers or behavioral targeting companies.

Contents 1 Web Bugs 2 Beacons 3 Cookies 4 Flash Cookies 5 HTTP Referrer

Web Bugs

Although there is a great definition on Wikipedia I'm going to tweak it for ease of use...

Web Bugs are programs that are loaded onto your computer by webpages. Some are used to make the webpage work better, some are used to track what you are doing, and some even report what you are doing to a server across the Internet.

Beacons

A beacon is usually an image on a webpage, but could also be an in-line frame, or even a web bug. The critical part of the beacon, is that it causes your computer to request something from a server on the Internet. As the request goes out to the server, cookies might get sent telling information about you, or the request itself might include information that helps the distant computer figure you out.

Cookies

Cookies are small chunks of text that are sent between your computer, and a server on the Internet. This exchange happens whenever a web page you are browsing needs another part, such as an image or video.

Cookies work very simply:

1. Let's say you are reading your friends blog, and on his blog page he has a picture of a sunset.
2. When your computer first draws the blog page on your screen, it asks for the picture of the sunset from a server, say www.coolpictures.com/sunset.jpg
3. When the server www.coolpictures.com sends back the image sunset.jpg, it can also send a cookie along with it. For example the cookie might have the text "likes sunsets".
4. The next day you look at a webpage that includes another picture from www.coolpictures.com

Flash Cookies

Locally Shared Objects (or LSO's) are also known as 'Super Cookies' or 'Flash Cookies'. They serve much the same purpose as a normal, text-based cookie but bring certain advantages that the 'traditional' cookies don't have. For example:

• Traditional cookies have a limit to hold up to 4kb worth of data where as a LSO's can hold up to 100kb.
• Traditional cookies have an expiration date while LSO's do not.
• Traditional cookies are usually stored in different locations, by different browsers. Super cookies are platform independent meaning that the same cookie can be accessed by Firefox, Internet Explore, Safari -- even Adobe AIR applications!
• Traditional cookies are text based, so they can sometimes be read or understood. Flash based ones are more difficult to understand and could be lightly encrypted.

Some flash cookies have legitimate reasons. For example, a flash game might store a high score or other data in a flash cookie much the same way a website might store your user preferences in a traditional text based cookie. Also, just like the traditional text-based cookies, a flash cookie can only be accessed by the site that sets the cookie.

HTTP Referrer

A (HTTP) referrer is the means to inform a server, the page or address from which you last visited. Here's an example:

1. You perform a search, lets say from Google.com about "Computer hardware"
2. Some results pop up, and you decided to click a link that leads to www.Newegg.com
3. After landing on Newegg.com, your browser sends it's referrer - "Http://www.google.com" to the "newegg.com" servers.

This is not to be confused however with other information that may be present in your address (URL) bar that appears as well. A HTTP referrer is a separate concept. Also, a referrer is only the last page, not say 2 or more address back.

Some sites use referrers to prevent hot-linking while other say it improves security (Dubious - up for debate). After all, if you login to say your bank, wouldn't the bank want to make sure you actually went to the right page from the beginning?

However, while the HTTP referrer may have some 'legitimate' reasons for its use, it's often a way to credit/inform advertiser which links/ads are working and which ones are not. After all, clicking an ad will inform the site you land on where you came from, Which ever ad is getting the lowest traffic is probably not the best placement. You can visit This page to see the HTTP referrer in action.