Clonazepam is a benzodiazepam, just like Xanax.  Klonopin (the brand name) is metabolized slower and may last longer due to a longer half life.  It's used for panic attacks and also for seizures.  Xanax is typically prescribed for anxiety/panic attacks.  both affect the same neurotransmitter in the brain, but doctors sometimes find that patients respond to or tolerate some drugs better than others.  So when it comes to these conditions, you  might try one and not like it or not respond to it or not be able to tolerate certain side effects, so the doctor will try another drug from the same class.