What Is the Difference Between 1080p and 1080i?

Resolution
Both 1080p and 1080i have 1080 horizontal lines of vertical resolution which with a widescreen aspect ratio of 16:9 results in a resolution of 1920 × 1080 pixels (2.1 megapixels). It is not true that 1080i has a lower vertical resolution than 1080p.
Frames vs. fields
1080p is a frame-based or progressive-scan video where you are dealing with frames. You have frame rate and it is expressed in frames per second. 1080i is a field-based or interlaced or interleaved video where you are dealing with fields. You havefield rate and it is expressed in fields per second. A field contains half of the lines of the frame, either even lines or odd lines, and if one field is composed of even lines, then the next one will be composed of odd lines and so on.
Frequencies
1080p has a frame rate of 25 frames per second for TV in PAL countries, 30/1.001 frames per second for TV in NTSC countries and 24 frames per second for cinematography. 1080i has a field rate of 50 fields per second for TV in PAL countries and 60/1.001 fields per second in NTSC countries. (Note that it is not 30 frames and 60 fields per second for NTSC but actually 30/1.001 and 60/1.001which is approximately 29.97 and 59.94 but the difference is important. Read about the NTSC color encoding on Wikipedia to see why.)
How to think about it
1080p at 25 frames per second: Imagine that you are shooting 25 pictures per second and storing them as bitmaps. Every frame is a full picture from the given instant. Every pixel in that frame was captured at the same time.
1080i at 50 fields per second: Imagine that you are shooting 50 pictures per second but storing onlyhalf of the bitmaps every time – sometimes you store the odd lines and sometimes the even lines. (Note that it is not the same as storing pictures with lower vertical resolution.) Every field is a half of a full picture from the given instant. Every pixel in that field was captured at the same time.
50 halves is not the same as 25 full pictures
Interlaced video at 50 Hz does not mean that 25 full pictures per second are shown. It means that 50 halves of pictures are shown but those are halves of 50 different pictures that were shot at 50 distinct moments of time in every second. You not only don’t have 50 full pictures per second – you don’t have any full pictures at all.
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