Azathioprine is an imunosuppressant. Which means it suppresses your immune system.
As sweetmelody said - most docs start first with the mesalamines (Asacol, Colazal, Lialda), if those don't work then they will jump to a taper of prednisone (steroids) and then on to imuran(azathioprine)/6mp (immunosuppressants) and even Remicade/Humira (biologics).
The mesalamines are prefectly safe for long term use. Most people never get off them - they stay on these for maintenance. Prednisone is not usually recommended for long term use - it causes many unpleasant side effects. Azathioprine is used for long term maintenance for those that cannot achieve remission with just the mesalmines. They also have some side effects but seem to be okay for long term use as long as you go for monthly blood tests and are monitored by a doctor.
What meds are you on now?
Mamas Boys (aka KTM)
Colazal, Azathioprine, Remicade
Culturelle, MultiVitamin, Folic Acid