Bash is a free shell for Unix-like operating systems from the GNU Project.
A shell is a command line environment that allows advanced scripting. It is provided on many (all?) operating systems based on or inspired by Unix. Notable OS's are Mac OS X, BSD, Solaris and the different flavours of Linux.
Bash is such a shell created in 1987 to fix some of the problems of the older Bourne Shell. The Bash command syntax is a superset of the Bourne shell command syntax and almost all Bourne shell scripts can be executed by Bash without modification. Bash stands for Bourne Again Shell.