Bash is a command line shell for Unix and scripting programming language well suited to writing glueware. Unices where it isn't available usually have a similar language such as zsh, fish or ksh. It was designed as a replacement for the Bourne Shell (hence the name Bourne Again SHell) by Brian Fox and released in 1989 for the GNU Project.
Bash is normally used interactively in terminals or through prewritten scripts.