Bash is the GNU project's Bourne Again SHell. It's the de facto standard shell on most GNU/Linux distributions, and therefore the first shell that most newcomers to Unix/Linux tend to see these days.
Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2).