A biodata module (COMIC: The Promise) was Time Lord technology which, when used in conjunction with a Chameleon Arch, could modify the biology of an individual, essentially changing that individual's species while a TARDIS could program the individual's memories to allow them to fit in better.
The module, often shaped like a fob watch or a similar object, was placed on the front on the Time Lord's forehead and stored the Time Lord's essence. When opened, it would return it to the Time Lord. (TV: Human Nature / The Family of Blood) The Eighth Doctor claimed that destroying a Chameleon Arch while the user was not a Time Lord would render their Time Lord personality and biology irretrievable. (AUDIO: One Life) The biodata module was incorporated with a perception filter to ensure that it would not be noticed. (TV: Human Nature / The Family of Blood, Utopia)
Modules resembling fob watches were activated by opening the lid. (TV: Human Nature / The Family of Blood, Utopia) The module containing Ruth Clayton's true identity, appearing to be a fire alarm, was activated by breaking its glass. (TV: Fugitive of the Judoon)
The Ninth Doctor gave a biodata module to Plex, modified to only copy the user's mind, without giving a new one. It was later retrieved by the Twelfth Doctor, destroyed and without any remaining biodata. (COMIC: The Promise)