The obvious way to connect two LEGO pieces is by putting the studs of one into the underside of another. But there are other ways. For example you can use a right-angle bracket to connect bricks at a 90 degree angle. For example, the headlights or taillights on most LEGO cars and trucks are often attached this way. In the AFOL (Adult Friends of LEGO) community, the term for this is SNOT (“Studs Not On Top”).
But the trouble with most SNOT techniques is that the bracket pieces take up bulk that sometimes you can ill afford in your model. One of the techniques featured on Unique Brick Techniques a few weeks ago is a very compact way to make a 180 degree connection between two plates. It seems to have been discovered by Brickshelf user “kerouac” (whose full identity I have not been able to find). Read more about it in the threads on Classic-Castle.com and LUGNET.