What
A test case design technique for a component in which test cases are designed to execute >branch outcomes.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST)

Branch outcome in the above could also be decision outcome.

Branch Testing is a structural or white box technique, because it is conducted with reference to the code. A decision is an executable statement that may transfer control to another statement.

In an ideal world every branch of every component would be fully tested. However, in the real world this hardly ever happens.

In branch/decision testing every possible branch is tested, regardless of the outcome. Compare this to Statement testing, where if there is no statement on the branch, it is not tested.

Why?

   1) To find faults
   2) Give credible information about the state of the component, on which business decisions can be taken.

Who?
Idealy an independent person or body, to the one that wrote the code. Usually, however it is conducted by the developer.

Where?
As close as possible to the point where the code was written.

When?
As part of component testing.

How?
Each test case, the following should be specified:-

   1) Inputs to the component
   2) Decision or branch to be executed
   3) Expected outcome.

A metric branch decision coverage is available to measure how much of the code has been executed.