NEB is a Massachusetts-company
that is the leader in the isolation and production of restriction
enzymes. Their website has a number of excellent resources related
to restriction and other DNA/RNA modifying enzymes. Every laboratory
scientist I know has a copy of the NEB catalog--it's just that
useful!
This page from New
England Biolabs gives an overview of restriction enzymes,
specifically their role in nature and the three types of REs
(Type I, Type II, and Type III).