The method used to telecine films to NTSC video. Film runs at 24 fps, but NTSC video runs a 30 fps (or 60 interlaced fields per second). To get these to numbers compatible, film frames are captured for 2 and 3 video fields alternating.

Film:   1 2  3 4  5 6  7 8
Video:  11222334445566677888

It means that NTSC DVDs run at the same speed as the original film, but it sometimes introduces an unpleasant jerking in slow panning shots.

Just to get anal retentive, here are a few amendments to Leynos' writeup:

NTSC video actually runs at a frame rate of 29.97 frames per second. Therefore, during telecine (and, hence, to make 3:2 Pulldown actually work), the film passing through the telecine machine is slowed to a frame rate of 23.976 frames per second.

This causes a problem: to get a precise matchback to your negative after cutting a film project on video (i.e., in a digital non-linear editing system), you must edit at 24fps. Otherwise, your negative cutter has to sync up your video cut and the film negative in a lockbox and check the edits by eye...and they're gonna be slightly off.

Therefore, when digitizing telecined footage (if you want to cut at 24fps...not necessary for video-only release), the video must be sped up by 1%. Digital NLEs have a 'pulldown' function which provides this solution.