Hd vs sd channels????????????

Guys, I have dishnetwork and was wondering the difference between hd and sd channels. Hd seems more clear than non hd channels but when I check the input source on my plasma tv, it shows both as 1080i resolution. I know that's not correct because there is picture quality difference between the two. I use hdmi cables. But when I connect my Sony DVD player, the input source in tv shows as 480i, connected using component cables. The only difference I noticed so far is hd has Dolby sound and sd has PCM . So what's the real resolution of hd and sd?