For mouses also dpi, which is the speed at which your cursor moves. 800 dpi is casual and works fine for small screens, 1200 dpi is good for most situations and average for shooter games. 2000 dpi rocks all the way.
EDIT: don't pay more than €20 for a mouse, it's a waste of money.
Also never buy wireless. Why? You can lose the USB thingy and wireless also tend to lose contact after years of usage. Wired mouses don't need a battery change once in a while.
Keyboards? 100+ accessoires you don't even need. The old, keyboards with big smash keys are more comfortable than you think and also extremely cheap. Sometimes even free.
