In 1080i, only half of the pixels are working at once and they alternate every 1/32 of a second. In 1080p, all of the pixels are working at once. They both have the same resolution (amount of pixels), but in 1080i only half of them are working at any certain moment. You can barely tell the difference but if you watch a lot of sports, 1080p is the way to go. On the other hand, 1080i tv sets are a lot cheaper.

Jake is right but before deciding on which HDTV to buy you should know that most sporting events are broadcast at 780i. This reduces the amount of bandwidth needed and reduces latency. Unless you have an HD receiver and the specific event is broadcast in HD, there really is no benefit to buying a 1080p for watching sporting events.