When you are playing online poker you have the ability to purchase software known as Heads-Up Displays (HUDS). These HUDs provide you with a statistical view of your own, and your opponents, games to help you make optimal decisions. One of the statistics that is often overlooked is Aggression Factor (AF). The AF measures how often a player bets or raises versus how often they call. For the purpose of AF calculations checks are ignored.
The AF is expressed as a ratio and takes card play from all four betting rounds into consideration. The formula used is very simply aggressive acts divided by passive acts. A player with an AF of 0 or 1 is a very passive player while a player with an AF of 5 or 5 is very aggressive.
To understand AF in isolation is a risky business. Instead it is advised that you balance your AF with your VPIP (Voluntary Put into the Pot). Imagine you have a VPIP of over 50%. This means you are playing a very wide range of hands. Now if your AF is 4 or 5 it tells you that you are also raising with a wide range of weak hands. Now if you compare this to a VPIP of 15% and an AF of 4 or 5, you can see that you can be aggressive but with a much narrower range of hands. This is vital when deterring counter strategies against your opponents.