Mountains cause moisture-carrying winds to rise. The clouds dump all the rain on one side of the mountain range, causing lush vegetation. On the mountain range’s other side, there’s no water left to fall. This causes a “rain shadow,” an area that receives little to no rainfall. Deserts are the usual result. This process plays out across the Earth. A large list of examples can be found here: http://www.artofworldbuilding.com/rainshadow.
Rain shadows often occur near a coast and can cover half a continent, such as in the United States. Desert-like conditions occur closest to the mountains, though this is less severe farther from the mountains that are causing the rain shadow, partly because moisture doesn’t come exclusively from the oceans and will be picked up in the atmosphere to fall as rain (albeit less of it). As we progress farther from the mountains, desert may give way to grasslands and finally forests.