Function

Relation: If A and B are two non empty sets, then A cross B is a set and the subset of A cross B is called relation.

Function:If a belongs to A and b and b' belong to B and (a,b) and (a,b') belong to A cross b then it(A cross B) is a function if b=b'. That means function is a kind of relation where two ordered pair would not have same first element in its pairs but a function can have same second element.
That means if (a,b) and (a',b) both belong to A cross B then A cross B could be said as a function.
f={ (0,1) , (55,22), (3,-50) }
g={ (2, 1) , (5, 2), (2, 9) }
h={ (1,7) , (1, 7), (33, 7), (32, 7) }
Here f, g and h are three relations. But f is a function and others are not

For help: http://www.mathwarehouse.com/algebra/relation/math-function.php