Pixel Aspect Ratio
Although Pixels themselves don't have height or width, being simply points in space, they do sometimes belong to frames where a single pixel represents a non-square portion of the screen. These are referred to as non-square or rectangular pixels. If each pixel represents the same amount of height and width it's considered square. In either case, the pixel is said to have a Pixel Aspect ratio (PAR), representing it's width to height ratio. For example, NTSC DVD (4:3) video has a PAR of 10:11, while in Widescreen (16:9) it's 40:33. PAL uses a PAR of 59:54, with 118:81 for widescreen (16:9).
PAR isn't used for display purposes. Display is determined by the video's Display Aspect Ratio (DAR), which tells the decoder what shape an entire Frame is. PAR is instead used for editing purposes, where image editing software often needs to know the shape of each pixel to produce a specific shape in the frame. For example, in order to draw a simple circle it's first necessary to determine how far apart each pixels is both horizontally and vertically, which can be determined by the PAR.
Some video stores a separate PAR value, in addition to a DAR for video decoders to use. Other times it must be calculated from reading the DAR first. You may be using editing software capable of this, but it's generally recommended to know your source video's PAR (or calculate it yourself) in order to double check any applications automatic settings.
Standard computer monitor resolutions use square pixels, as do standard HDTV resolutions. EDTV also uses square pixels, although digital SDTVs use pixels with the same PAR as NTSC or PAL DVDs.
Resizing DVD-Video To Square Pixels
Digital Video Fundamentals - Resolution and Aspect Ratio
Related glossary terms
Related software tools
AVIcodec (Open source)
AVIcodec is a nice little tool for identifying what codecs you need in order to play your multimedia files. Program supports MPEG-1/2, AVI, Ogg, ASF, WMV, etc formats.
GSpot is a nice little tool that allows you to see what codecs your video file uses in order to determine what you need to install on your system to watch it.
MediaInfo is a free software that supplies technical and tag information about a video or audio file.