What is the purpose of Software Requirement Specification?

Software Requirements Specification (SRS) is a primary document for development of a Software. It is written by Business Analysts who interact with client and gathers the requirements to build the software. It goes through severl reviews through out the software life cycle.

SRS is the main part of Documentation...

it contains all Steps to be done in Documentation of Software Development.

There are many steps like Market Survey, Analysis,Design, Functional and Structural Behaviour, etc...

Just see the Software Engineering Book , by Pressmen.