Check out this page on the history and organization of British and Empire orders and decorations:
http://www.regiments.org/biography/orddec-h.htm
There's a generic history of medals at
http://www.diggerhistory.info/pages-medals/history.htm
Information on the world's most famous medal, the Victoria Cross, can be found at
http://www.victoriacross.net/default.asp
Mr Mephisto