Restriction enzymes belong to a group of enzymes called endonucleases. They are also called restriction endonucleases or restrictase. These enzymes recognize specific sequences on double-standard DNA (called restriction sites) and cleaves both strands resulting in fragmentation or linearization (in case of circular DNA such as plasmids) of DNA. Depending on the type of restriction enzyme, the cleavage site can be located within the restriction site or can be present near or far away from the restriction site.
Restriction enzymes are naturally found in bacteria and archaea and are part of a defence system, called restriction-modification (R-M) system. The R-M system destroys invading viruses DNA while keeping the bacterial DNA safe from restriction endonuclease action by modifying it using modification enzymes such as methyltransferase that methylate recognition site, thus protecting bacterial DNA from deleterious effects of restriction endonucleases.