What's the difference between a Cohen and a Rabbi in judaism?

A cohen can be a rabbi but a rabbi unless he is born a cohen from the line of Aaron can not be a cohen, a cohen is a descendent of the children of Aaron who was the 1st preist among Israel who had the charge of officiating the animal sacrifices in the temple and is the only one that may enter into the holy of holies once every year, no other Israelite can have that service other then the cohen.