Relation is a subset of Cartesian product .
Let us take an example
Let A = {a, b, c, d}
and B = {Aman, Ankit, Baljinder, Chandu, Eklavya}
We define relation as
R = {(x, y): x is the first letter of y, x ∈ A, y ∈ B}
So, our relation R is
R = {(a, Aman), (a, Ankit), (b, Baljinder), (c, Chandu)}
Representing relations with arrow is called arrow diagram
Now, in our relation
Domain = Set of all first elements in R
Domain = {a, b, c}
Note that
Domain ≠ A
Range = Set of all second elements in R
Range = {Aman, Ankit, Baljinder, Chandu}
Codomain = 2nd set
Codomain = Q = {Aman, Ankit, Baljinder, Chandu, Darshan}