Given two sets X and Y, a relation between X and Y freely associates elements of X with elements of Y, with no restrictions.
A function is a relation with some restrictions: there must be exactly one element of Y connected with each element of X.
The set X is called the domain of the function, and it represents all the possible inputs that we can feed the function with. As we just said, every element of the domain must have a correlated element in Y.
The set Y is called the range of the function, and it represents all the possible outputs that the function can return.
