What is the fundamental difference between a relation and a function ? Is every relation a function ?

Function is a type of relation. But in a function no two ordered pairs have the same first element. For e.g. : $R_1$ and $R_2$ are two relations.
Clearly, $R_1$ is a function, but $R_2$ is not a function because two ordered pairs (1, 2) and (1, 4) have the same first element.
This means every function is a relation but every relation is not a function.