Desert

The Desert is a sandy area that spawns desert monsters, cacti, and waterleaf. Each map will generally contain at least two desert areas. Drained Oceans also become deserts. As of update 1.2, pools of water in the Desert biome take on a sandy brown color. Pyramids are rarely created during world generation in Desert Biomes.

Hardmode
At hardmode Desert can be corrupted or hallowed. Both of these will be very similar to their main biomes (e.g. The Corruption or The Hallow): the music theme is the same, the mobs are the same (excepted the Dark and Light Mummies), and the background is just slightly different.

The sand will be replaced by ebonsand or pearlsand. Although their color will change, cacti still drop cactus when cut, and waterleaves are also able to grow on hallowed desert.

Making money
If a player is desperate for money, deserts can be a good indirect source. Sand blocks can be crafted into Glass, which can be crafted into Mugs, which are filled at a Keg to produce Ale, which has a sell price of 20 Copper Coins each. Though the sell price is low, this is one of the only sources of coins in the game that is always readily available, as all other abundant block types cannot be crafted into sellable items without other ingredients.

Assuming the player carries only a Pickaxe and one other Weapon for fighting off any enemies encountered, while keeping all other Inventory and ammo slots available for sand collection: 42,000 sand blocks (42 slots x 1000 max sand stack per slot) = 21000 Glass blocks = 21000 Mugs = 21000 Ale x 20 Copper Coins each = 420,000 Copper Coins, or 42 Gold Coins.