Dashboard Widgets + My Mac Desktop aren't enough!

As it happens, there are some terrific programs available that can solve the exactly problem you're talking about, and they work by creating "virtual" desktops. With one of these programs installed on your computer, you can easily have two, three or even four or more different Mac desktop windows, with different content and application windows in each.

On my laptop, for example, I have four windows, labeled "Firefox", "Safari", "Entourage" and "Editing/Research", with the applications you'd expect running in each.

The specific program I use is a freeware application called Desktop Manager and it's really a splendid add-on, something so useful that I think it should just be included with Mac OS X, actually.

While it has lots of features, I run it in a stripped down minimalist mode, so the only clue I have it's running is that I have a few extra icons on my menubar:

Here you can see that I have four workspaces defined, I'm working in #3 (it's blue), and that I also have apps running in workspaces #1 and #2. Oh, and this is managing both my displays as I have a dual display setup.

At any time I can move an application's window from one workspace to another with the menu that pops down from the two-box icon too:

It's really quite easy to use this application, but even if you just define two workspaces, say 'school' and 'fun' or 'work' and 'play' or whatever, then I'm sure that it'll accomplish exactly what you seek.