FOr example, in linux you can boot to a terminal, you don't have to load a desktop, window manager, etc. Is there something like this in Mac OS X? And like in Linux, is it possible to switch to one of these from in the desktop?
I don't think Single User Mode is really what you want.
A better idea is to use a console login.
To do this, first you have the login window set to show “Name and Password”:
Then you can login as ">console" as the username (no password) and get a terminal prompt.
It's not very pretty, but it's handy if you need it.
In addition to what TJ Luoma said, you can also:
Go to the normal login screen, with the list of users.
Select one (any, it doesn't matter) with the keyboard.
Press ⌥ Option+↩︎ Return.
>consolein the username field, leaving the password blank.
Press ↩︎ Return and log in.
Unfortunately I haven't had much luck with this—sometimes it just doesn't show up, with or without an external display.
On power up hold the 's' key down. You'll enter single-user mode where the interface is via a full screen terminal window (the console).
There are caveats, you are essentially root at that point, and yes you could switch user or execute login.
To switch to a console from the desktop just use Terminal.app and maximise it so when you switch to it is the full screen.
Note that this is not the same as Single User Mode as you will be logged in as a user, so it is like xterm under Linux.
Just note that Linux and OS X are different and will do things different ways e.g. from the Linux console boot you can start X11 (i.e. the desktop) but not in OS X.