What is a script language?

There are various definitions, but typically a script is an interpreted (rather than compiled) computer language, and it is usually designed to work with operating system activities rather than actual data processing. So you write "programs" or "applications" with (say) C++ or Ruby, but write "scripts" with (say) bash or Applescript.

There is a lot of crossover though, with javascript (note the name) used much for web applications (and you can even do a little data processing with operating system commands in bash).

(bash = Bourne Again SHell - the user interface between linux (usually) and the outside world.)