Video can be interlaced or non-interlaced. When a video signal is interlaced, each video frame is made of two fields of image data. Each field is a collection of every other scan line in an image, starting with the first or second scan line. The first field, referred to as the odd field (or field 1), contains the data for the first scan line and skips every other scan line to the end of the image. Similarly, the even field (or field 2), carries every other scan line starting with the second. The "even-ness" or "odd-ness" of a field is referred to as its field polarity.
When video is not interlaced, each field contains all of a frame's scan lines. Typically, video signals are sent at a rate of 30 frames per second; in the case of interleaved video, this means the rate is 60 fields per second.
The fields that make up a frame do not always reflect the same moment in time. For example, if the frames are separated by 1/30 of a second then the two fields of a frame may be separated by 1/60 of a second. Because a television displays each field individually, no two fields are simultaneously visible, and the difference between fields adds to the illusion of movement.