If you only play games on your PC every now and again, you can get away with a normal cheaper mouse. If however you go online and play against other gamers, getting a good gamer mouse will give you a significant edge. In addition I found that when I switched to a decent mouse for playing games the greater control over the game made it much more fun for me.
logitech gaming mouse
Speed and precision
These are the 2 main features that differ significantly between a normal and a mouse for gamers. The speed of a mouse is measured by the time between updates, as to it's state, to your PC (also called response time). For a gaming mouse it should be 1 ms. The precision with which a mouse can detect movement changes is measured in dpi, for a gaming mouse it should be more than 3,500 dpi.
Gaming mice basically come in one of 2 designs. You have your smaller rounder mice that are intended to be used with your fingers and then you have larger flatter more ergonomic mice, on which you can rest the palm of your hand. I would recommend the smaller mice for those who play games that requires precise control for short periods, for example first person shooter games like Call of Duty. I would recommend the more ergonomic mice for gamers that log long ours playing games like World of Warcraft.
Wireless gaming mice
If you asked me a few years back I would have said that the response time on a wireless mouse is too long, if you want to play games use a cabled mouse. Fairly recently however the wireless technology has improved and it is now possible to buy a very fast wireless gaming mouse. It does give you more of a sense of freedom as you aren't as tied in to your computer. Be warned however these devices are quite a bit more expensive than the wired kind.
Durability and portability
A gaming mouse gets banged around quite a bit, more than an everyday mouse. If you are going to take your mouse with you to gamer events or internet cafes, you will need a carry bag. Some of the mice come, with their own. Also some gaming mice have internal memory to store your settings. That means you can plug it in and use one of your profile's settings without installing any new software.