Everyone seems to have you on the right track for Partial, Standard and Free Actions. Full Round Actions have been glossed over, though.
Again, Full-Round Actions are not a measurment of time, it is an action type.
When an action is described it will say whether it is a Standard or a Full Round action, so read the descriptions and check the tables.
If you take a Full Round Action that is all you can do in that round. You cannot move your speed, or take a move-equivalent action. If your Full Round Action does not include movement, you can take a 5ft. step (before during or after the action itself).
The three most common Full-Round Actions are:
1) Full Attack Action. This is where you get to use multiple attacks (and the only action that allows multiple attacks). If you are fighting with two weapons, or have 4 attacks due to a high base attack bonus you must take a Full Attack Action in order to use the extra attacks.
2) Spontaneously Cast Metamagicked Spell. If a Sorcerer or Bard casts a metamagicked spell it takes a full round action to accomplish. The spell resolves on the same initiative count it was started (unlike casting a spell with a casting time of "1 full round").
3) Run. If you want to run all-out at 4X your speed (3X in heavy armor, 5X with the Run feat) you must do so as a Full Round Action. Again, all of your movement is resolved on your initiative order.
Cheers.
PS There is also a type of action called "Not an Action: Some activities are not even considered free actions. They literally don't take any time at all to do and are considered an inherent part of doing something else." (SRD). So the example above of Knocking an Arrow is actually not an action; it is considered to be an inherent part of firing a bow. Likewise, retrieving material components for most spells is considered "Not an Action", as it is an inherent part of casting the spell.
Enjoy.