Here you can find a list of all commands available in the simulator, what they do, and how to use them. There are two different sets of commands: aircraft commands and scope commands. To toggle between entering aircraft commands and scope commands, use the backquote (`) key.
As you might have guessed, aircraft commands are the commands used to issue instructions and communicate with aircraft. Many commands have "aliases" which act as abbreviations of the full command word, thus saving you time and effort. For example, AAL123 to will work exactly the same as AAL123 takeoff. You may chain as many aircraft commands in a single transmission as you like.
Scope commands are what controllers type into their scopes for things like handoffs, moving data blocks, changing brightness settings, and similar tasks.