All this data is potentially out of date, and should be taken with a truckload of salt
Interdictors are tech 2 ships based on the destroyer hull of each race. Much like a destroyer the interdictor has a host of highslots intended for frigate sized guns making it very dangerous for any frigate. But that is not the interdictor's primary role. Instead they are the only ship that can fit and use a bubble launcher. This modified probe launcher fires a Warp disruption probe that creates a bubble effect with a 15km radius around it. Inside this bubble effect no ship can enter warp, not even supercapitals, nor can capital class ships jump out.
Interdictors are one of the most useful ships in any large fleet engagement making them very high on the primary list. Combined with how easy it is to kill a destroyer hull Interdictors are often dubbed "flying coffins". None the less, being so incredible useful they are flown in great numbers on the big battlefields. Some of the big alliances hand out free interdictors to anyone foolish enough to fly them in big fleet fights.