[the new JR Jansen]

Well, in BG, every combat was basically fought in real-time. If combat started, you paused and you could assign orders to your party members and when when you unpaused, everything would be played out in realtime again.
In turn based combat, you don't need to pause. Combat starts and everyone (ie your party and the opponents) each gets a certain score derived from one, or a combation, of their stats. This is the initiative role. After these rolls have been made, wich is done by the computer so it's instantanious, the character with the highest, or lowest depending on the system, number gets to act first. He makes an action and then the following one gets his turn and so on untill combat is finished. If you didn't use up your action points in one turn, you can choose to end your turn. The 'lost' action points aren't really lost because they are added to something else, in the case of Fallout, to your armor class.