Mac Game Store is the official Apple store for video games, that functions in a very similar way as Steam or Desure, offering an impressive game catalog through its client.
This application will allow you to purchase lot of games from all genres, ranging from the biggest releases from large companies such as Assassin's Creed: Brotherhood or Deus Ex: Human Revolution, to independent titles such as Aquaria or Tiny and Big in Grandpa's Leftovers.
Of course, in order to play them, you will have to buy them first, but once you have them, they will stay in your virtual game library forever, allowing you to download them at any time and on any computer.
The Mac Game Store games catalog cannot compete with the other large distributors for PC, but it does come with some of the most important titles for its platform.
Mac Game Store is a fantastic alternative for playing on a Mac. Although it has to compete with a giant like Steam, it certainly sets the stage for what should be a good video games platform.