A brief (and incomplete) explanation of springboard groups.
I didn't want to clutter trs's excellent tip thread with this, but it came up so I figured I'd post a new thread.
Springboard Groups
Springboard groups are a handy way to group springboards in order to add another dimension of control over how they interact.
By default, each new springboard is set to a "default" group. This is what causes activating one springboard to deactivate all the others by default: they are all in the same group.
Some themes have sections that stay on even when you activate others. This is done by putting those different elements in different groups. I'll use screens from a theme I've been working on to demonstrate.
Here is the base layer with only 3 springboards visible on the left: Home, Comm, and Sys. They are all in the same group (main) so activating one will deactivate the others.
Activating one of the springboards on the left brings up 3 more on the right. These 3 are in a different group: in this case, "comm". Hitting any one of the Comm options will deactivate the others... but the original 3 buttons on the left will NOT be deactivated.
Then hitting a different main button (Sys) deactivates (Comm), its associated buttons and their content to give you 3 of its own-this time in the group "sys". (Cntl) is set to be ON by default so its content comes up.
Since Apps, Cntl and AUX are in the same group (sys) they will deactivate each other to cleanly switch content. And since they aren't in the "main" group they won't deactivate you main options.
Other Observations
Group behavior persists across landscape and portrait modes. If your portrait uses some (or all) of the same elements as your landscape theme, even if they are arranged differently, then this can be a good thing. You'll be in the same environment in both orientations. If your landscape and portrait layouts are different, though, be careful with your springboard groups. If they different for each orientation then you can leave portrait in one state, work in landscape and make state changes there, and return to portrait in the same state you left it.
If your layouts are different for each orientation and your groups are the same... it possible to need a few taps to get back to where you want to be when you change orientations as activating any springboard in one orientation will deactivate all of the springboards on the other orientation.
Groups add a great degree of control of how whole sections of your theme work and interact with each other.