Magic is built around its Core Sets. There are really three ages of 'Core Sets.'

Bronze Age (1993)

This goes from Alpha up to Unlimited. These three sets were identical in their composition though they varied in card forms. This was the formative age of Magic and up to this point these sets all held the hotly desired 'Power' cards. Moxes, Black Lotus, Time Walk and others were all printed in these three sets.

Silver Age (1994-2009)

Spanning a decade and a half, it is the majority of Magic's history. Starting with Fourth edition the core sets were released every two years and were composed of a number of different cards from previous core sets and cards pulled the expansions. All of these core sets (except for the last one, Tenth edition) have white borders and a numeric expansion symbol.

Golden Age (2009-)

This age dawned with the release of Magic 2010. With this set Core sets would now follow the new naming structure "Magic XXXX" where XXXX is the following year. So "Magic 2010" was actually released in 2009, and Magic 2011 will be released in June of 2010. These core sets also are introducing new cards never before printed. Some of these new cards are noted to be 'functional reprints' such that they are the same as older cards but under new names.