An interlaced video system is one where, in general, two interleaved fields are used to scan out one video frame. For a two field system (e.g. PAL, NTSC, SECAM) the number of lines in a field is one half the number of lines in a frame. The odd numbered lines are contained in one field and the even numbered lines contained in the other. Each field is drawn on the screen consecutiveley - first one field then the other.
The reason for using interlaced video are concerned with frame rate. A large TV screen that was updated at 30 frames per second would flicker, meaning that the image would begin to fade away before the next one was drawn on the screen. By using two fields, each containing one-half of the information that makes up the frame and each field being drawn on the screen consecutively, the field update rate is 60 fields per second. At this update rate the eye blends everything together into a smooth, continuous motion.
It is very important to note that the odd and even fields that make up a single frame are not temporally equivalent. Thus an interlaced image of a scene that included motion would exhibit tearing when displayed statically.