Mac
There are two ways that Mac poker players can play at online poker rooms. Larger poker rooms usually offer a dedicated mac client that allows players to download the poker room to their computer (similar to what a windows user would do). The other option is various poker rooms offer a no download version of their poker room which allows you to play directly through your internet browser. This allows anyone to play at the poker room including mac, windows or linux users.
Most players like to play at mac poker rooms that offer a mac download option because it usually has more features and usally runs a bit better. Popular poker rooms that have a native mac download are Full Tilt Poker and recently Poker Stars.