MoonCoins are bought for real money and can be used on any game you might playing on the MoodID platform (e.g. Monsters Army, Monsters Game, Knight Fight ...)
To get ahead really fast in this game, Days of Evil, you might want to convert those MoonCoins to Souls as they give you access to a lot of extra goodies.

To do that, you just have to go to the Soul Dealer and chose your package of souls. If your money is tight, you might want to wait for special occasions when you get something extra for the same amount of real money / coins.
Except for buying Moon Coins you can get them by playing a game on MoonID portal. It is called "Play and Win". The rules are on the game website. There are 20 winners every week. First 10 players, according to the highscore ranking, get different amounts of Moon Coins and another 10 lucky winners (randomly picked) get 600 Moon Coins.
I hope this helped.
Cheers,
st0rm