Time signatures

Time signatures help count the rhythm of written music.

They come in a whole range, here's a brief guide to the common ones:

Simple time:

 2/4 - count 2 per bar
 3/4 - count 3 per bar
 4/4 - count 4 per bar

Compound time:

 6/8 - count 2 per bar
 9/8 - count 3 per bar
 12/8 - count 4 per bar

A few others:

 common time - count 4 per bar, as 4/4
 alla breve - count 2 minims per bar
 cut time - count 2 per bar, as 2/2

And more:

 5/4 time - count 5 per bar
 7/8 time - count 7 per bar
etc