Variable Framerate

Video that has an inconsistent Framerate is called Variable Framerate, which may be abbreviated as VFR. Few containers support this, with Matroska being the notable exception.

VFR video can be used for a variety of purposes, from combining video with both PAL and NTSC framerates to encoding both original film and film that's been edited as NTSC video (after being telecined) with the highest quality and most efficient Bitrate possible.

Since modern HDTV hardware and braodcast standards support an array of framerates there may be increased interest in VFR video in the Matroska (MKV) Container, particularly since it's popular for storing video from HDTV captures.

Matroska (MKV) Related Guides
How to play MKV files

How to play MP4 files

How to play RMVB (RealMedia Variable Bitrate) files

How to play mkv files with x264 video and AC3 audio on the Sony PlayStation 3

Other Guides
Digital Video Fundamentals - Frames & Framerates