Question

what are restriction enzymes

Answer 1

These are enzymes which cut double stranded DNA at specific sequences. They come from bacteria and they are part of the defence against viruses that use bacteria as hosts (They are called bacteriophage). Restriction enzymes are also known as endonucleases - ie they cut the DNA in the middle rather than at the ends.
Restriction enzymes can be used to clone DNA sequences - to put the sequence of interest into a vector so it can be grown up and introduced into other organisms - what is known as recombinant DNA technology. I hope this helps but if you have any more questions please let me know.