A **browser fingerprint** is a collection of info about a browser.  When you visit a website, you send a variety of information, like screen size, installed fonts, and software versions; the site can also ask you for much more, as demonstrated by <a href="http://www.browserspy.dk">BrowserSpy</a>.  Like human fingerprints, browser fingerprints are usually unique.  With your browser fingerprint governments and advertising companies can track you across the web, making a list of the things you've looked at, the places you've been, and what you've said and done.  

**Linkability** is the ability to link two or more profiles, identities, or pieces of data. For example, imagine you have a Gmail account and a Facebook account; you have them registered under different names, and believe nobody can trace them back to you, or to each other.  In reality, the advertisers and government agencies have linked you to both of them by using your browser fingerprint to follow you across the web.  

The Tor Browser Bundle helps to prevent this kind of attack by making its fingerprint the same as everyone else's -- among other things -- thus making it more difficult to track you, identify you, and link your various identities to one person.  This is, like most things, imperfect.  Most websites use other companies to host their data and offer services and ads.  Every time you visit one of these sites, you are by extension visiting all the others.  This leaves a very messy footprint; after visiting just two sites, you may actually have notified dozens.  There is a <a href="https://addons.mozilla.org/en-US/firefox/addon/collusion/">browser extension</a> that makes a very neat visualization of this.  A good way to avoid this is to block content: installing AdBlock would prevent ads from being downloaded, and thus those companies/sites would not be aware of you.  The same goes for Flash, JavaScript, and even images.

Interesting links:

<a href="https://panopticlick.eff.org">Panopticlick</a>

<a href="http://www.browserspy.dk">BrowserSpy</a>

<a href="https://en.wikipedia.org/wiki/Device_fingerprint">Wikipedia Article</a>

<a href="http://noc.to/">Device Fingerprinting</a> (Under the JS Display menu, cursor position is among the reported data.)