Currently Being ModeratedApr 27, 2012 6:26 PM (in response to lisapstevens)
I assume you desktop is on your MBA.
Open the Displays preferences and look at the Arrangement. It should show you the two displays (MBA and TB display). You can slide them around to produce the spatial relationship you want. The little white menu defines the main monitor, i.e, where you menu bar is located. You can drag that to the other monitor if you want it to be the main monitor.
The thing that may be confusing you is you have a choice of running mirrored or in extended desktop mode. I suspect you have extended desktop, i.e., the additional monitor provides additional space for your desktop. You can move you mouse and windows across the two monitors in the "space" defined by that spatial relationship you set in the Arrangement tab.
If you click the mirrored checkbox (lower left corner of the Displays preferences) then both monitors will be identical. Everything on one is on the other, i.e., they are "mirrored". The problem with mirrored is (a) the max screen size on both monitors is determined by the smallest screen in the set and (b) in my opinion it's such a waste of screen real estate.