What do interlaced and progressive scanning mean?

The 2 most common picture formats currently used for HDTV are:

a) 720p - where p denotes progressive and the image is composed of 1280x720 pixels
b) 1080i - where i denotes interlaced and the image is composed of 1920x1080 pixels

In an Interlaced format, the screen shows a picture in two fields with each field scan every 50th of a second, and then follows that up with the even field in a second scan. Thus the entire picture is scanned every 25th of a second. For smaller screens, this is less noticeable. As screens get larger, the problem with interlacing is the flickering.

Progressive scanning shows the whole picture, every line in one showing, and every sixtieth of a second. This provides for a much smoother picture, but uses slightly more bandwidth for broadcasters.