Not all relations are function but all functions are relation. A good example of a relation that is not a function is a point in the Cartesian coordinate system, say (2, 3). Though 2 and 3 in (2, 3) are related to each other, neither is a function of the other.
Function is a relation between two variables that inhibits an apparent connection. If the variables are x and y, then y can be determined for some range of values of x. We call this, y as a function of x denoted by y = f (x). Differential Calculus is limited only to those relations that are functions defined by equations.