Shell

A shell is an interface with which we can interface with a program. It refers to the main executable file of a program, to which we can pass commands.

For GUI applications, usually we launch them from their icons, but we can also edit the default launch command to set additional parameters.

For example, we can edit the launch command of a game's shortcut file from game.exe to game.exe --windowed to make the game run in windowed mode. Or something like --enable-cheats or --draw-distance=max.

Often a GUI has a settings section to edit these values, but essentially, the shell is the program that reads the settings and commands and executes the program.