Bonsai tree

This is an old Satsuki azalea, with its massive trunk and wild display of mixed up flowers.
Ramification just means branching. More specifically in bonsai  it is sometimes used to mean branch development or branch refining.

Pruning

Pruning is a commonly used word that is loosely applied to mean any removal of all or parts of branches. More specificaly it means removing branches rather than shortening branches.

Thinning

Thinning means removing unwanted branches or twigs.

Trimming

Trimming usually means shortening branches or twigs or even new shoots, but is sometimes used to refer to removing them.
 
With any bonsai it is necesary to thin on a regular basis. Branches that grow into or crowd other branches, branches that grow in towards the center of tree, parallel branches that grow too close to each other, branches that are too thick for where they are on tree, unhealthy branches and etc, all need to be thinned.

Sometimes branches grow too long and need to be shortened. In this series of illustrations, first they are thinned and then what is left is shortened.
The technique of simply cutting a branch wherever you want works with azaleas and some other types of trees but will not work with conifers. The conifers you need to cut at junctures, or just above buds or nodes. You also need to leave some living growth below the cut.