2. what is project plan?
A document for management purposes that gives the basics of a project in terms of its objectives, justification, and how the objectives are to be achieved. This document is used as a record of decisions and a means of communication among stakeholders.
As I said earlier, S/W Engg. By Roger S Pressman can be of big help for you to understand about S/W Process. Like Testing, Phases etc.
what is documentation? what is its purpose?
Documentation is to provide comments on what you did code. In simple words you write comments on your code, so that others can easily understand what your code is doing.
Suppose today you made some code & some body else is asked to enhance something in that code...at that time your documentation can be of great help to the one going to use code.