NTSC and PAL are *BROADCAST* standards, they're not truly present on any DVD. Your DVD player decides how display the information on the discs, depending on your setup. "NTSC" and "PAL" DVDs are designed to be most compatible with the capabilities of the hardware it's thought to be played on, but they're not really NTSC or PAL, but yes, they are often referred to as such.

Wikipedia:

The term "PAL" is often used informally to refer to a 625-line/50 Hz (576i, principally European) television system, and to differentiate from a 525-line/60 Hz (480i, principally North American/Central American/Japanese) "NTSC" system. Accordingly, DVDs are labelled as either "PAL" or "NTSC" (referring informally to the line count and frame rate) even though technically neither of them have encoded PAL or NTSC composite colour.

They're Not Really PAL or NTSC
The first thing I need to clarify about DVD is that PAL and NTSC are words and formats that are applied to DVD for convenience, and because of historical convention. There is nothing fundamental about a DVD which makes it either PAL or NTSC.

At their heart, DVDs are merely carriers of data files with compressed audio-visual information contained therein. This information can be placed on DVD in (usually) one of two resolutions; 720 x 576 pixels (PAL DVDs), or 720 x 480 pixels (NTSC DVDs), and with various frame rates (~24, 25, and ~30 frames per second are common). The DVD player itself takes this data file and formats it appropriately for display in either PAL or NTSC.