There are 3 grenade types in Halo 4 (frag, plasma, and pulse). Frag and plasma return from previous games, but they have not behaved exactly the same in all previous installments. Pulse is completely new for Halo 4. As I cart around my duffle bag full of grenades, in what circumstances does each type excel (or are some just better than others)?
Frag grenades are on a timer, and explode after several seconds, regardless of the surfaces they contact or where they roll. The advantage of the frag is that you can bounce it off of surfaces, and therefore you can do indirect damage to enemies that are around a corner. You can also get a bit more distance between you and a target, as they will bounce and roll on flat terrain.
Plasma grenades stick to surfaces, and then detonate shortly afterwards. This can make them good against enemies that tend to dodge when grenades are thrown at them. If you connect with the throw, it's a guaranteed hit, and in most cases a guaranteed kill. However, without the ability to bounce, they're a bit shorter range and can't be used as effectively for indirect fire.
Pulse grenades are the new Promethean grenade type. When thrown, they detonate when the hit a surface. However, they will spend several seconds creating a shield- and health- draining sphere with a fairly limited radius. It's about as tall as a Spartan, Elite, or Knight. After a couple of seconds, they explode, doing significant damage. Since they're a bit more area-of-effect and damage-over-time, they're good for when you know you have an enemy pinned or stunned. The sphere's area of effect also slows vehicles caught within it.