Extra Maps and Extra Modes

You can choose to play these different modes on the title screen. In Extra Maps you can choose any Map you want to play on and go on whichever difficulty level you want. Extra Modes are different you choose a special level and a difficulty level. Basically it's a bit like a normal challenge but could have only one type of Creep in it. For example Spawn is where you face bunches of Green Spawn Creeps.