Erangle was the first map of PUBG, its size is 8x8KM. The loot can be bad sometimes, and the gameplay is slow compared to some of the other maps.
Miramar was the second map of PUBG, its size is, like erangle, is 8x8KM. It takes up more of the 8x8 it was given then Erangle did. The loot can be bad as well as Erangle, but it seems to be better loot than Erangle.
Sanhok was the third map of PUBG its size is 4x4KM, the loot is typically very good. You are able to get any gun you prefer. This map has much faster combat, making it easier to learn the game.
Vikendi was the fourth map of PUBG, its size is 6x6KM. You would think the loot would be very balanced, but it seems they have not got it figured out as you typically get shotguns and SMGS.