If you've used a Linux system, you know that when you log in, you are greeted by a prompt that looks something like this:

$

The particular prompt that you see may look quite different. It may contain your system's hostname, the name of the current working directory, or both. But regardless of what your particular prompt may look like, there's one thing that's certain. The program that printed that prompt is called a "shell", and it's very likely that your particular shell is a program called bash.