Yoga comes in 3 basic flavors:
Meditative- Hatha, this is more the pose and breath and pose and breath, not too fast or you'll hurt something. Called heart yoga and deals with balance and self awareness. Includes shadow yoga
Hot medium paced yoga- good for balance and strength building and detox. This includes vinyasa, Forrest, and a few other new ones. Also called flow yoga its not too difficult on the joints.
Power yoga- Ashtanga, yogalites, and power yoga. This is hatha yoga on steroids, usually with heat. Its high impact and while its great for strength training and focus you will jostke your joints especially shoulders and knees.
Assuming its a full hot class: Expect to sweat like you've not sweat before, expect to use muscle groups you forgot you had, be open minded: yoga is based on the precepts of chakra energy and some instructors get pretty pointed on this and don't be afraid to fart b/c you will most likely need to at some point.
have fun, dont take it too seriously and drink lots of water afterwards.