Mac OS X ships with DVD Player and does what its name implies, it plays DVDs.  Note that you must have a DVD-ROM drive (combo drive or SuperDrive) installed on your Mac to use DVD Player.

As with other Mac OS X applications, the settings that dictate how the application functions are stored in an area called Preferences.  To see or alter the DVD Player settings:

You can use the DVD controller to perform actions or control the Mac OS X DVD Player.  You also have the option to use your keyboard and mouse to perform tasks and actions.

DVD Player will support just about any DVD you purchase, given the appropriate region restrictions.  DVD producers use region encoding to restrict where a DVD can be played by storing information on a DVD disc.