AF - Total Aggression Factor - Poker Tracker

I've just started using poker tracker and think its really going to help my game, i've always put it off because i'm not very good at using these sort of things but i've found it easy to understand and use. The only thing i don't really get is the AF or Total Aggression Factor, what does it take into account? and is the number a percententage? Obviously the higher the number the more aggressive the player is, but i just wonder what stats it uses to get the number and what you can read into it.

I only ask because i realize that it is an important stat and i see it used in many a hand analysis', any explanation you guys could give would really be appreciated, thanks.

AF = (Raise% + Bet%) / Call%