The following statements explain the difference between relation and function,
1.  Function has a special with output and input.
Relation is a mapping of input and out put.
 2.  In a function all inputs have only one output.
 In a relation a input can have as many outputs as it wants as long as all inputs have outputs.
 3.  In difference between relation and function, the relation is somewhat like a entire function problem and with input and output.
 4. In difference between relation and function, the function is merely a particular relation with accurately one output for every input.