What's the difference between a simple and a compound time signature?

Mathematically, 3/4 and 6/8 have the same numer of crotchets and quavers per bar. So why is 3/4 simple and 6/8 compound? Well, the difference is how you divide the bar. 3/4 has one strong beat, and will usually have 3 crotchets in a bar. 6/8 is compound time because it has 2 strong beats in a bar. The 2 strong beats come from the bar being made up of 2 groups of 3 quavers. So it is a compound of these two groups of quavers, hence, compound time.