Bash is the shell from the GNU project. It is the standard shell on many Linux distributions and often available on other *NIXes.
Bash (the Bourne again shell) is a Unix shell. It was intended as a free replacement to the Bourne shell and includes many scripting features from ksh. Bash is intended to conform to the POSIX 1003.2 standard.
Bash also includes more advanced interactive features such as command line editing with the readline library, command history, job control, dynamic prompts and completion.