The classical restriction enzymes cut up, and hence render harmless, any unknown (non-cellular) DNA that enters a bacterial cell as a result of a viral infection.
They recognize a specific DNA sequence, usually short (3 to 8 bp), and cut it, producing either blunt or overhung ends, either at or nearby the recognition site.
Restriction enzymes are quite variable in the short DNA sequences they recognize.
An organism often has several different enzymes, each specific to a distinct short DNA sequence.
[1] The list includes some of the most studied examples of restriction endoncleases.