You must differentiate between the various actions taken.
First, there is the movement. It generally does provoke an AoO, if you leave (not enter) a threatened square. However, a 5' step (if that's your only movement for the round) never provokes an AoO.
Then, there are various actions, that do provoke AoO's if performed in a threatened area. Spellcasting, attacking with a ranged weapon, etc. - the various actions are all listed in the PHB.
So, you might even provoke multiple AoO's during the course of a single action (one from your movement, one from your standard action taken), but each opponent can only take the chance to attack you once per round.
Bye
Thanee