The first letter of the name comes from the genes and the second two letters come from the species of the prokaryotic cell from which they were isolated.
Like EcoRI comes from Escherichia coli RY 13. In EcoRI, the letter 'R‘ is derived from the name of strain.
Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria.
Restriction enzymes belong to a larger class of enzymes called nucleases.