A great part of this site deals with interlacing/deinterlacing which introduces some of the nastiest interlacing problems like these: Do you think you record 25 frames per second when you make a movie with your digital camcorder? Your digital camcorder does the following: Records 50 pictures per second, intermixing every 2 consecutive pictures (with half the height) into 1 frame. Now these two fields are mixed (=interlaced) into Frame1 (full height): What you see above is an exact frame as on tape of your camcorder.
Here is a zoomed view of the above Frame 1: As you can clearly see above, Frame1 consists of Field1 and Field2.
This can be done by comparing frames by time or by space/position.
This gives you good results in quiet scenes where not much is moving, because there is nothing blurred then.
The main reason for this: Movies with 25 non-interlaced (=progressive) frames per second don't look very fluid.
If you watched a football game with 25 progressive fps it would look as if the ball isn't flying fluidly thru the air.
Thus in fact you reduce the quality down to a quarter of the possible quality.