What are the current legal tournament formats for Magic: The Gathering? What are the differences/definitions of each format? At what point do sets rotate in/out of a format?
You can visit the Banned / Restricted Lists for DCI-Sanctioned Magic: The Gathering Tournaments page to see a list of formats, plus deck construction rules and a list of banned/restricted cards for each format.
With the exception of prerelease Limited, new sets enter their appropriate formats on the day of their official release. The specific dates for upcoming sets are spelled out in the "New Releases" section of the most current Magic Tournament Rules (PDF link).
As for a description of each format, plus differences and defining characteristics, is probably overly broad as a question.
Note: While it's not a tournament format, Commander does have rules listed in the CR. Cards are legal in Commander as of their set's prerelease.– Brian SNov 19, 2013 at 17:04