(i) one - one
(ii) many - one(iii) one - many
(iv) many - many(i) one - one
For each element of B there is one corresponding element of A

(ii) many - one
For an element of B there is more than one element of A.

(iii) one - many
For one element of A there is more than one corresponding element of B.

(iv) many - many
For one element of A there is more than one corresponding element of B and for one element of B there is more than one corresponding element of A.
The relation is called 'onto' if every element of B has a corresponding element of A.
For example, Figures (i) and (ii) are onto relations.The relation is called 'into' if every element of B does not have a corresponding element of A.
For example, Figures (iii) and (iv) are into relations.