Bourne shell script
A Bourne shell script is a text file intended to be executed as a set of commands for the Bourne shell, which is a shell (command line interpreter) for Unix-style operating systems. While a number of other shells exist for such operating systems, the Bourne shell (and descendants such as Bash) has been the normal default since Version 7 Unix in 1977, so it is what most computer people think of when discussing "Unix commands".
A shell script is the Unix-style OS (Unix, Linux, OS X, etc.) version of what is known as a batch file in DOS/Windows systems, but has more sophisticated programming constructs, permitting very elaborate logic.
Shell scripts will typically have a .sh file extension and #!/bin/sh (see Shebang) as their first line (pointing at the path to the shell, of which /bin/sh is the traditional value, often symlinked to an actual program path somewhere else in the directory structure).