In a 4/4 time signature, a whole rest lasts for four beats, a 1/2 rest for two beats, a 1/4 rest for one beat and an 1/8 rest for half a beat. An eighth rest is half the length of a quarter rest. In other words two eighth rests make up a quarter rest, while four of them make up a half rest, and eight 1/8 notes make up a whole rest.
Explanation:
I hope this helps a little bit - I just learned it a few weeks ago. :)
