The six segments of the key to time are:

* A lump of Jethrik crystal taken to Ribos by Garron (DW: The Ribos Operation)
* The pillaged planet Calufrax, now shrunk down to football size by the Zanak. (DW: The Pirate Planet)
* The former Great Seal of Diplos, kept in the form of a necklace on Earth by Cessair. (DW: The Stones of Blood)
* A Tara statue. (DW: The Androids of Tara)
* A holy relic of the Swampies, swallowed by Kroll on a Delta Magna moon. (DW: The Power of Kroll)
* Princess Astra of Atrios. (DW: The Armageddon Factor)

The Doctor was sent by the White Guardian to recover the segments of the Key to Time so the White Guardian could restore balance to the universe.

There was a subsequent series of audio dramas where the Doctor was again sent to recover the Key to Time segments that had taken different forms.
Source(s):
http://tardis.wikia.com/wiki/Key_to_Time

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