Bash is the Bourne Again SHell - the default command interpreter for Linux systems. And a scripting language.
GNU is GNU's Not Unix - the open source project to clone (and improve) proprietary Unix utilities. It was supposed to be a whole system but they proved incapable and that's why we run Linux.
Anyway - 'man bash' and 'http://gnu.org' (I think - the site won't load for me at the moment, so I can't verify the address.)
-- Oops. Scooped. Yeah - www.gnu.org.
*That* one loads. *g*