Endonuclease are enzymes that cleave the phosphodiester bond within a polynucleotide chain. Restriction Endonuclease cuts at a specific nucleotide sequences. Endonuclease differ from Exonuclease which cleave at the ends of recognition sequences instead of middle,endo, portion. 

A restriction site is a pallindromic sequence about 4 to 6 nucleotide long. These are sequences which is similar when read from left to right and right to left respectively.