AKA yoga with brawn, it's the American interpretation of Ashtanga yoga. Many of the poses resemble basic calisthenics like push-ups and handstands.
Power yoga is a general term used in the West to describe a vigorous, fitness-based approach to vinyasa-style yoga. Most power yoga is closely modeled on the Ashtanga style of practice. The term "power yoga" came into common usage in the mid 1990s, when … see full wiki
I've been going fairly regularly to power yoga for the past ... week. I've gone every once in awhile prior to that and was always happy with the classes. Going more regularly is like an added bonus - I'm stretchier (more limber if you want to get technical), happier in general (go endorphins!), stronger, and better equipped to deal with the little day-to-day annoyances I encounter. The heat sucks yet is awesome at the same time. I know this sounds odd, but it makes a difference. Doing some of the poses and flowing into them would be a lot harder if the air wasn't already warming your body and muscles up. You may sweat a lot, but I'd take that over pulling a muscle. Considering I used to play sports, this type of yoga fits best with the mentality I've developed as an athlete - pushing yourself (while maintaining balance with relaxation, of course), 'no pain no gain' (I'm sore after every class - in a good way), and I can phsyically feel myself getting better and stronger.