Never heard of XP Antivirus 2008, or it's cousin Antivirus 2009? You haven't gotten constant popups claiming "Your Computer is Infected!"? Well you've been lucky so far, and if you don't know what I'm talking about, read further on about the world of RansomWare.
The typical characteristics of RansomWare is that they masquerade as either a virus alert, scanner, or removal tool for your computer. You will get various popups and browser windows opening up unsolicited on your computer, with messages telling you about various problems it has identified on your machine, usually with a "click here to fix" attached to it.
Clicking on these is about the worst thing you can do, at best you will be taken to a screen giving you the hard sell to buy a product you don't need, and at worst you will be downloading the virus, trojan or worm itself onto your computer.