The sequence whose terms follow certain patterns is called progression. 

There are mainly three progressions which are : 

1. Arithmetic Progression (AP): A sequence is called AP if the difference of a term and the previous term is always same.

Example: 2, 4, ,6 ,8, 10,............

Here the difference of any term and its preceding term is always 2. So it is in AP                                       

2. Geometric Progression:  A sequence of non-zero numbers is called GP if the ratio of a term and term preceding to it is always same.

Example: 2,4,8,16,.............

Here ratio of any term and its preceding term is always 2. So it is in GP  

1. Harmonic Progression:  A sequence (a1, a2, a3,.........) of non-zero numbers is called HP if the sequence 

(1/a1, 1/a2,1/ a3,.........) is in AP

Example: 1, 1/2, 1/3, 1/4,...........is in HP because 1, 2, 3, 4,.......... is in AP