MIB stands for the Mutant Investigation Bureau. MIB deals with crime that includes mutants. MIB agents are embedded with all major government syndicates, a definite point of contention between agencies..
In Hudson City the MIB maintains a mutant registry of all known mutants. Failure to register carries a $2,000 fine and up to 10 days imprisonment. Failure to register and committing a violent crime increases the fine up to $20,000 and an additional 5 years of jail time.
Due to the specific nature of their role MIB agents have access to equipment and training that allows them to deal with very specific threats.
Most MIB agents are humans and prodigies, a very small percentage are mutants, and even more rarely, aberrations.