One-to-One Function: A function f  from A to B is called One-to-One function

if f(a) = f(b) then a = b

Again no element of B is the image of more than one element in A.

Example: f(x) = x3 is a one-to-one function, where f:R->R 

Onto Function: A function f  from A to B is called Onto function if for every b in B, there is an a in A such that

if f(a) = b

Here all elements in B are used

Example: f(x) = 3x - 4 is a one-to-one function, where f:R->R