VINYASA/ ASHTANGA- most popular form of yoga in the west. vinyasa means flow, so the classes (depending on the level) will be pretty quick paced with poses linked together as combinations- most often using a sun salutation between sets. poses vary, depending on what part of the body is being worked that particular class. now- ashtanga is a flow class that has a set series of poses and levels that will be basically the same each class time. if you're looking for an active class- vinyasa rocks. at a mature studio there will be a beginning meditation which will help set the tone and the rhythm of your breath for the rest of your practice.
http://www.ayri.org/

IYENGAR/ HATHA- this is a more traditional form of yoga, being slow paced and uses props extensively. in some iyengar classes you may spend half of the class doing introductory poses to a particular back bend only to practice the backbend at the end of class once. Iyengar can be pretty technical, but its a great way to learn poses.
http://www.bksiyengar.com/

ANUSARA- the newest and latest craze. anusara is an even paced yoga class focusing on the heart as propelling force of every pose. i've only ever been to one anusara and it had a heavy focus on alignment, but was a v. interesting perspective.
http://www.anusara.com/

i think your best bet is to get out to a beginner class or two and try them out. every studio has its own purpose within each style. so its a trial and error mission. hopefully there's a few studios either near your house or your work that will proove beneficial and convenient and usually your first class is free. do some web lookin' and be sure and tell me what you discover~ feel free to contact me with any questions~ kaliyugayogaclub@yahoo.com