What exactly is "The Metasploit Project" ?

http://www.metasploit.com/

-----------------
The Metasploit Project

Metasploit provides useful information to people who perform penetration
testing, IDS signature development, and exploit research. This project
was created to provide information on exploit techniques and to create a
useful resource for exploit developers and security professionals. The
tools and information on this site are provided for legal security
research and testing purposes only. Metasploit is an open source project
managed by Rapid7.
----------------

What exactly does this software (Metasploit and Nexpose) do?

Can it act like a simple diagnostic scanner, telling the user what his
system is vulnerable to?

Does it run on win-9x/me platforms?
.