I think Ascension is right about the tropics, except for one point: it's not because of the sun baking things (although that's what makes it a HOT desert), but it's still because of rainfall... Basically, because of the Coriolis Effect, wind patterns are driving moisture to and away from certain places. In the case of the Tropics, wind patterns are driving moisture away from those lines and toward either the Equatorial region (where we see a lot of hot, steamy rainforests) or toward the Temperate regions above and below the Tropics.

The Gobi and the Desert Southwest are caused by Rainshadow.