From LXF Wiki
Bash is a Unix shell - a command line environment - where the user can enter text commands to perform tasks.
The name comes from Bourne Again SHell, after the writer of the Bourne Shell, Steven Bourne, on which it is based.
Bash is generally compatible with the Bourne Shell sh shell but extends it with command line editing, "job control", functions, aliases and various other features.
Bash is the GNU project's shell and has a page on the GNU project's site (http://www.gnu.org/software/bash/bash.html).
Other shells exist for Linux. See the shell page for a list. To view a list of usable functions in bash type help at the bash prompt or read the bash manual (type man bash at the prompt).