Ah Combat. It seems to be at the heart of almost every role playing game, and even those that try to focus on other things will generally have something to help resolve it. And when it comes to combat, one of the first things that most systems run into is needing a way to determine who gets to resolve their actions first. In other words, I want to talk about Initiative today.
In general there are two types of initiative: dynamic and static. I'm going to talk about each one of these in turn, and they each could be broken down into further types, but let's keep it simple for today - ok?
Dynamic Initiative is any system where a character's initiative changes from round to round. This could be as simple as rolling initiative each and every round - as some games say to do if you go by the book - or having characters initiative score change at some point during the combat round (usually the end.) Dynamic initiative is interesting because it can add strategy and more depth to combat. The chance to get two turns in a row if you manipulate your initiative well is powerful, and can make the difference between a victory and a loss - sometimes even a TPK. However, the system isn't without its weaknesses.
For one, and most importantly, dynamic initiatives add an additional level of complexity to the proceedings. This also tends to slow the game down. Either the players all have to re-roll their initiative every round, or some kind of book keeping has to be done. Either way, the order of actions changes each round and that can lead to confusion - especially in bigger games. Still, for some these down sides are more than worth the benefits you get out of it.
Static initiative is probably what most of us are familiar with. It is the bread and butter of most RPGs. Combat starts, you roll initiative, and that turn order is what you are going by for the rest of the fight. If that means that the Death Knight you're fighting always gets to go first, then that is what you are stuck with. Roll better next time.
The strengths and weaknesses of static initiative are basically the opposite of those for dynamic initiative. The key strength lies in making it easier to manage combat. There is less book keeping, less die rolls, and less need to figure out who goes when. The players can key off of each other to keep track of when they go, and the GM can use that same knowledge for playing the right NPC. After all, it is easy to remember "the goblin commander goes after Sharon" then it is to keep track of the goblin commander's initiative each and every round. On the down side though, the static placement of speeds can make combats feel a little flat and the loss of complexity can remove some strategic elements. Still, in larger games this is generally a good trade off.
The Third Option
I lied above, and didn't realize it until right now. there is a third type of initiative that isn't covered by Dynamic and Static: the tick system used by games like Exalted. In this system - which I have no direct experience with , tragically - each character has a speed rating. This determines how many ticks go by before each action. So a fast character may go once every 2 ticks while a slow character only goes once every 8. In other words, the fast character gets 4 turns in the time it takes the slow character to get only 1.
From all I have heard, this system works fairly well and combines a lot of the benefits of both static and dynamic as well as a bit extra. Still, others don't like it as it can suck being the slow guy, especially when the fight is over before you even get to go.