Field-based video is not composed of frames as film and stop-motion brickfilms invariably are, but composed of interlaced fields. A field contains half of the horizontal lines (either evens or odds) in a video signal, with the following field containing the other half.
Two interlaced fields are not equivalent to a single frame since they are temporally separated. This leads to many problems converting back and forth with frame-based video. A 3:2 pulldown is used to convert between 60 field per second and 24 frame per second video. 30 frame per second video may be converted to 60 field per second video by simply using each frame for two consecutive fields. Neither of these is a perfect conversion.
Television is broadcast using field-based video standards such as NTSC and PAL. Media destined for television such as DVDs must be aware of this fact as it places certain constraints on the video, and the format conversion may cause problems.