Assault ships v. Interceptors
Assault ships are a bit heavier and ment for "prolonged" fights. Have better tanking capability. Can deal a nice amount of damage as well as take in a nice amount of damage.
Interceptors are very light, very fast version of the tech 1 counterpart. Have very low tanking capability but rely heavily on speed as a "tank". Can deal a very nice amount of damage.
In proper hands these ships can be very dangerous. I think they were created to enhance wolf pack tactics.
You need to be aware that in some cases a more skilled pilot in a cruiser will be in your way. You have a great chance of dying.
The short answer is, the interceptor is for speed tanking and holding the enemy in place until someone comes and kills him. The Assaultship is for doing good DPS.
An assaultship can normally orbit a BS too close for the tracking of the BS to hit him. All the while doing a lot of Damage (for a frigate).