nick1302 - Jack and BEF- can y'all elaborate more? My brother has been trying to get me to a bikram yoga class.
Bikram yoga is a series of postures that are performed in a hot room. Each class is always the same postures, in the exact same order.
Ashtanga is similar in that it's the same sequence done each time, except that it doesn't need to be done in a hot room. Ashtanga is very active, in the sense that between most postures you do a sequence consisting of a plank, a lowered pushup, an upward facing dog posture, and then a downward facing dog posture. It also flows quite fast from posture to posture.
Then you will have power yoga and vinyasa flow yoga classes, which are often (at least in my experience) ashtanga based. This means that you'll do ashtanga postures, but not necessarily in the order you would do them in a regular ashtanga class. You will also do postures from the more advanced ashtanga series. Most ashtanga classes will only follow the primary series.
The slower paced yoga classes are often referred to as yin yoga. You can hold poses for up to five minutes in these classes, as opposed to five breaths forost ashtanga poses.
I'm sure there are other types, but this is what I have experience with. Best advice is to try a bunch of different classes and see what you like best, if you're interested in giving yoga a try.