I use strike indicators for 3/4 of my fly fishing or more.
Football indicators-
Good for about everything, and float extremely well
Yarn indicators-
Use these for small stream trout, with very clear water. They land extremely softly on the water when you cast, therefore not spooking the fish.
"Sindicator"-
These are a brand. They are basically round balls with a loop at the top. They come in three sizes. Personally, i prefer football indicators over these. When a fish takes a bait under the football, it goes down super easy, letting you know you have a fish, whereas sindicators don't react quite as well
There are other types, but those are the most common.
First one is a football, second yarn, and last is a sindicator. What rusty said is great.