Like terms : The terms having the same literal factors are called like or similar terms.
In the algebraic expression 12a2 – 15b2 + b2 –17a2 + 8ab + 9, we have, 12a2 and –17a2 as like terms and also – 15b2 and b2 are like terms. 

Unlike terms :  The terms not having same literal factors are called unlike or dissimilar terms.

In the algebraic expression 3p2q + 5pq2 – 7pq – 9qp2, 5pq2 and –7pq are unlike terms.