Combat in Wasteland 2 is turn-based. This means that the individual characters take turns to take their actions, while the rest are waiting for their turn. Who and when makes his actions is determined by the queue that you can see in the middle, atop the screenshot. The turn in the queue is allotted basing on the initiative of the character, which mainly depends on Awareness and Speed. How much you can do, during a turn, depends on the AP points. Shooting, reloading, using the first-aid kit, walking - all of these use up AP. The cost depends on such factors as the weapon that you are currently using, skillfulness with that weapon, or the combat speed, which determines how many meters you can walk on one AP. The character automatically ends their turn after they have used up all of the AP. If there are still some left, but you cannot, or do not want to use them up, click the End Turn button. By not using up all of the points, you can increase the number of AP available in the next turn - you receive an additional point if you saved 1 or 2 points, or 2 for each 3, or more, that you saved in the previous turn. This is especially useful if you are waiting for the opponent to approach you, because the points that you save up may accumulate to the number required to take an additional shot. Transferring of AP is especially useful if you are using heavy weapons, which require more AP to be used, and which often makes it impossible to move and shoot at the same time. You can also receive additional 2 AP, at random, thanks to Luck. Note that, unlike in Fallout, you control all of your characters (unless any defies control at a given moment). Time in which you can use your turn is unlimited so, you can consider your options as much as you want.
To make orientation during combat easier, there are visual aids. The blue field marks the farthest distance that a character can walk and shoot, in the same turn. The yellow field marks the boundaries of your move, with using up all the AP. The red circle marks the range of the weapon. Moreover, when shooting, you will notice a red field around your hero - it's the range of noise that your weapon generates. An important aspect, when you plan to eliminate someone without attracting others. Of course, all of these measurements depend on what you are currently carrying. Changing the weapon also changes the boundaries so, you cannot hope that with one gun active you will go as far as possible, switch weapons and will still be able to take a shot. This only works only in both of the guns require the same number of AP to operate.