1080i- (interlaced) uses two fields to create a frame. One field contains all the odd lines, the other field contains all the even lines. It scans 60 fields every second (30 odd and 30 even). The two sets of 30 fields are combined to create a full frame every every 1/30th of a second, which is a display of 30 frames per second. 1080i is the minimum resolution needed to enoy high def television.

1080p- (progressive) the lines are drawn in one at a time in sequential order. The entire single frame image is created every 1/60th of a second, which is a display of 60 frames per second. 1080p has the highest high def signal.

In short, 1080p allows twice the detail of 1080i, because it has double the frame rate per second, which results in higher detail, resolution and less flicker, which results in overall better quality.