1080i interlaced
1080p progressive

replicand is correct that fields is for broadcast television and progressive scan negates its use (that's my understanding). Web use doesn't need fields. Or if you're going to film you don't want fields.

Some video has come from TV has fields and ends up on YouTube, all the motion looking very nasty.

Fields will make motion look smoother as 60 images per second equals higher temporal resolution. 

Watch a camera pan at the movies and at a moderate speed it is unbearable to watch.
That same speed shot on video will look smooth thanks to fields.

There is a dark side to this story and that is frames (2 fields) have a dominance. If field dominance is inverted this will look very nasty as within every frame your video will play backwards, advance one frame in time and within that next frame play back. The result being that your shot will run backwards every 30th of a second while it is also playing forward. 

There is a lot more re: fields but if they are no longer needed for most media I am glad.

re: After Effects
Don't get me wrong, love, love, LOVE After Effects but I find it a PITA to edit anything more than a single effect shot. For doing effects shots, Wunderbar!