1080p and 1080i are both high definition formats...that display 1920 lines vertically or across the screen and 1080 lines horizontally or up and down the screen.

The difference is the way the component sends it's signal to your TV.

1080p ("p" stands for progressive) means that there are 1080 individual lines of resolution that are all displayed at the same time, 1/30th of a second. This resolution will produce the highest quality images, BUT, the TV has to receive a 1080p signal in order to produce the best picture. Most satellite and cable companies only send HD in 720p and your TV has to "scale down" the image. It's worth noting that some TV's scale down better than others.

Blu-Ray is 1080p and some satellite and cable providers have on demand in 1080p, but as of 2010 I don't know of anyone broadcasting 1080p.

A 1080i signal comprises 1080 horizontal lines but all the lines are not displayed on-screen simultaneously. Instead, they are interlaced (hence the 'i'), ie every other lines is displayed for 1/60th of a second and then the alternate lines are displayed for 1/60th of a second. So, the frame rate is still 30 frames per second, but each frame is split into two fields, which your brain then puts together subconsciously.

The practical difference is that 1080p will give you a much higher quality picture (the quality is noticeable, especially in Pixar animated films) and is better suited to live action, especially sports. TV's that use interlacing will tend to produce a blur.

ESPN puts it like this: 'Progressive scan technology produces better images for the fast moving orientation of sports television. Simply put, with 104 mph fastballs in baseball and 120 mph shots on goal in hockey, the line-by-line basis of progressive scan technology better captures the inherent fast action of sports. For ESPN, progressive scan technology makes perfect sense.'