The Linux shell—as noted by Linus in the preface—is just another process to the operating system. Its basic function is to interpret and execute commands entered at the keyboard.
Those commands might be functions built into the shell itself, like ls or mv, or they might be programs, or scripts, which can be run at the command line.
The shell program most commonly used in Linux distributions is called Bash, but there are several others available for use. The alternatives include Ash, Ksh, Sash, Tash, Zsh, and others. In all these cases, the sh at the end of the name stands for shell.
The shell does more than simply allow you to type a command. Modern versions provide features like command history, command editing, ...