Modern is the newest constructed format. Modern was created by Wizards of the Coast in the Spring of 2011 as a response to the increasing popularity of the Legacy format, which although popular proved difficult to access due to the high price of staple cards. Modern allows cards from the 8th Edition core set and all expansions printed afterwards.
Legacy decks may consist of cards from all Magic card sets, any edition of the core set, and all special sets, supplements, and promotional printings released by Wizards of the Coast.
Cards from expansions and special sets (like From the Vault, Commander, Duel Decks, etc) are legal in the Legacy format on the date of release of the expansion or special set.