Alright...I'll try to keep the technical jargon to a minimum here.
HD is short for High-Definition. In short, a high-definition video will have a better picture quality than the ordinary camera. The image will be clearer, and the colors will be more vibrant and saturated.
So why does it matter?
1. With video, light is vital. When the lights begin to dim, a high-definition video will be the only acceptable way to record. With a lower quality camera, it will not be able to utilize the little light available, and it will result in ugly, grainy video.
2. After getting married, it's mandatory that the groom received a new 60" plasma TV (...or at least it should be). Well to maximize your viewing experience, you'll want to have a high-definition video to play.
3. With high-definition, you can have your wedding put on a standard DVD, a high-definition DVD, or a blu-ray disc (for the ps3 that goes with the plasma TV).
4. With technology changing so fast, you'll want to have your wedding documented with the latest technology currently available. Period.
Just to give you a visual reference of the difference HD makes, check
out the chart below: