Hot yoga is supposed to 1) be a cardiovascualr benefit because of the heat; 2) make you more flexible than you would otherwise be; and 3) release toxins through sweat.
These are mostly BS reasons, and are mostly to get girls/guys to think they are doing more of a workout than they actually are doing because they are sweating more. Although there is some science behind just heat cardio and you are able to stretch more in a hot room than a cold room, but in some ways it's not a good thing because you can stretch to much. You should be generating your own heat.
There is regular hot yoga and bikram yoga which has a specific sequence of poses you do and unlike most yoga has mirrors.
I like yoga as a relaxing counterbalance to running. I don't use it as a workout, but as an escape. It's also fun to try and do some of the tricks. I did not like doing hot yoga as I thought I was just going to die in there (as I'm usually a little dehydrated and running hot from the daily running I do and sweating profusely in regular yoga).