Aggression Factor (AF) = (Bets + Raises) / Calls
(checking/folding doesn't affect it)
This results in a value between 0 and INF (infinite)
If a player never calls, but always bets/raises, his AF will be infinite.
Both Aggression Factor and Aggression Percentage are only used in postflop decision making. It will tell you the likelihood the opponent will bet on each street. Harrington, in Harrington on Online Cashgames favors using the AF stats for each street separately in the HUD. (not relying on the 'overall' postflop AF stat)
Look at it this way: AF
<1.0: player DOES NOT BLUFF. He only VALUE BETS, he only CBETS when he has HIT, he only RAISES when he has hit the board (or has an overpair), etc.
1.5-2.5 Player will often CBET even when he's missed the flop, or will occasionally bet a DRAW, but he won't do anything crazy
3+ Player will: always cbet, bet AND is even capable to raise his draws, occasionally donkbet, probe bet, double barrel, ABUSE his position, BLUFF when a scare-card hits, capable to BET a missed draw on the RIVER, etc. etc.