There are two basic ways for a moving video image to scan it's constituent images (loosely, it's frames):
interlaced, whereby every other line of pixels is scanned, alternating, until it reaches the bottom of the frame and then returns to the top to scan the alternates - each of these alternating scans is one field
or
progressive, whereby every line is scanned in order.
Note that neither of these terms describes how a film frame is presented - the entire image simultaneously. Sometimes this too is referred to as 'progressive,' though the terms technically are reserved for video imagery.