Testing : What is a test plan?

A software project test plan is a document that describes the
objectives, scope, approach and focus of a software testing effort.
The process of preparing a test plan is a useful way to think through
the efforts needed to validate the acceptability of a software
product. The completed document will help people outside the test
group understand the why and how of product validation. It should be
thorough enough to be useful, but not so thorough that none outside
the test group will be able to read it.