I think 'Dinner' is more used in British-English and 'Supper' is more used in American English. Although hopefully both are as tasty as the other :))

the answer is in here ^_^ http://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1535_questionanswer/page2.shtml

I think dinner is the main meal of the day, you could take it in the middle of the day or in evening. Supper on the other hand, is the last meal of the day, it is usually less formal and substantial than dinner. Hope it helps:)))

On top of what everyone has said, from experience I know that some areas of the south (southern states in America) supper is the main meal of the evening (regionally). Dinner is the word that might not be used often. The state I know for sure that this is the case is Georgia (USA.) There are more, yet this is the one I can say with certainty.

Elisa - If you got anything from these answers, please explain to me. Now, I don't know when should I eat my meals? :D