Clenbuterol is NOT a steroid.  It is an beta 2 receptor agonist. It is prescribed as a drug for asthma, but this same mechanism is responsible for increasing metabolic rate and increasing lipolysis (the burning of fat).  Albuterol is another example of this, but it has a much shorter half life in the body than clenbuterol and is much more tolerable.

You can read more about Clenbuterol here:
https://web.archive.org/web/20060223105102/http://www.bodybuilding.com:80/fun/catclen.htm
https://thinksteroids.com/steroid-profiles/clenbuterol/
https://en.wikipedia.org/wiki/Clenbuterol