The Linux kernel supports virtual consoles. These provide a way of making your single screen and keyboard seem like multiple terminals that are connected to the same system. Thankfully, using virtual consoles is one of the simplest things about Debian: There are ``hot keys'' for switching among the consoles quickly. To try it, log in to your system and press Alt-F2 (simultaneously press the left Alt key, and F2, that is, function key number 2).
You should find yourself at another login prompt. Don't panic: You are now on virtual console (VC) number 2! Log in here and do some things - more whoami commands or whatever - to confirm that this is a real login shell. Now you can return to virtual console number 1 by pressing Alt-F1. Or you can move on to a third virtual console, in the obvious way (Alt-F3).
Debian comes with six virtual consoles enabled by default, which you access with the Alt key and function keys F1 through F6. (Technically, there are more virtual consoles enabled, but only six of them allow you to log in. The others are used for the X Window system or other special purposes.)
If you're using the X Window system, it will generally start up on the first unused virtual console - probably VC 7. Also, to switch from the X virtual console to one of the first six, you'll have to add Ctrl to the key sequence. So that's Ctrl-Alt-F1 to get to VC 1. But you can go from a text VC to the X virtual console using only Alt. If you never leave X, you won't have to worry about this; X automatically switches you to its virtual console when it starts up.
Once you get used to them, virtual consoles will probably become an indispensable tool for getting many things done at once. (The X Window system serves much the same purpose, providing multiple windows rather than multiple consoles.) You can run a different program on each VC or log on as root on one VC and as yourself on another. Or everyone in the family can use his or her own VC; this is especially handy if you use X, in which case you can run several X sessions at once on different virtual consoles.