PokerStars is a client for the PokerStars online gaming site. It allows you to connect to the PokerStars servers directly from your Mac, without the use of a web browser. From what I can see, all the game modes supported on the website are supported using this client. You can get into Hold'em games for real money or fantasy money. You can also play Omaha or Stud games using this app.
If you haven't registered with PokerStars, you can do it from within this app. It is a quick process, but they ask you quite a few questions to make sure you are a real person and that you are over 18. Once, you are registered, you can simply take a seat at any of the available tables. To do this, you can simply go to the mode you want to play and find a game. When you click on a game's name, the main window is minimized and a graphical representation of a poker table pops-up. If there are any open seats, you can just join the players and start playing. The game is identical to the web version from here on, and a good understanding of Poker could come handy.