The standard Human weapon, more or less, you'll start most missions with one of these in hand. Unfortunately, it probably won't stay there for long; the assault rifle is one of the few weapons in Halo that isn't really terribly useful for anything.
Well, not to disparage it - the assault rifle isn't dreadful. It has a large clip and very high rate of fire. It looks rather nifty, and includes a suave on-board computer that will show your state of ammunition as well as an electronic compass (for keeping your bearings, not for navigating - "North" or "East" have no meaning on a ring-shaped construct, and you'll never need to know them). Ammo tends to be abundant, as nearly all the Marines you'll see will be using an AR.
That's pretty much all, though. The one main reason you'd use one of these is for the high rate of fire; however, it's not the most accurate weapon, and to maintain any kind of spot-on shooting, you'll have to fire in short bursts. Which is sad, since the individual shots do very little damage; you need to really pound out a lot of them to do very much, and that's impossible unless you're within kicking distance of your target. In most cases, what you'll end up doing is an unfortunate "spray-and-pray" kind of behavior, and that's not something that's pleasant to have to suffer. Plus, shielded enemies are quite resistant to the assault rifle - compared to, say, a plasma rifle against an Elite on higher difficulties, the assault rifle is like shooting spitballs.
There are uses for the AR, of course. Primarily, on easier difficulties and on earlier levels of the game, it is often sufficient if you use it like a shotgun: run right in your opponent's face (melee distance, or close to it), squeeze the trigger, and don't let up until they fall. Also, it's a very efficient Grunt-sweeper. And there's times when you simply must have a good rapid-fire, instant-impact weapon, and there really isn't any other options.
Also, in Multiplayer, the assault rifle can be put to good use for just that - assault. Two or more streams of AR fire can form a devastating crossfire, and the simple fact that you have a fast rate of fire, a fair amount of ammo, and shots that don't hang in the air like pudding can make the AR surprisingly handy. And it fills a number of niches - e.g. range (more than the shotgun, less than the pistol), power (has less punch than the plasma rifle, but won't overheat), etc. Learn whether or not you like it, learn what situations it's good for, and put the knowledge to effective use.
It's actually surprising the learning curve that the assault rifle seems to sport. It's a very easy weapon for beginners, simple to use, and plenty effective on lighter difficulties. Progress some, and you may find that you're ditching the assault rifle as soon as you can for... well, anything else. But, continue on your journey to Mastery, and it's quite possible that you'll discover that it isn't so bad after all.
But it does require practice, if you're going to be using it on higher difficulties, anyway.
Melee speed is fairly quick.
Pros: Fast firing speed, large clip, amusing "UT" style weapon display.
Cons: Very little accuracy, unimpressive damage, mediocre melee speed.
Usage: Good for cleaning up small targets (Grunts, for example, and it's easily the most effective weapon in the game against the Flood). The most important thing is to get as many shots as possible on the target. Handy in Multiplayer. Otherwise, unless you find your tone with it, treat it like the needler - only to use if you really know how.
For: Some people like it... others loathe it. You'll know which in about ten seconds, although be wary of fickle opinions. Also a good weapon for beginners - the automatic fire and large targeting reticles make it simple and easy to use.