The katra, or living spirit, is the essence of the Vulcan mind and can be transferred to another person moments before death.
The existence of the katra was controversial, to say the least; many outside of Vulcan thought it was nothing more than a myth.
Legend has it that ancient Vulcans used katric arks to store katras. Some of those arks were discovered at the P'Jem monastery centuries earlier. Although they were analyzed by Vulcan scientists (one of them even tried to meld with an ark), they were not found to contain a katra.
When a katra is transferred to someone, he or she will benefit from the experiences of the person the katra came from. Because the katra can resist its transfer, the procedure is not without risk and must be performed according to a specific ritual. Non-Vulcans might experience side effects like a form of multiple personality. Humans are especially vulnerable because of the severe shock a transfer can give to their nervous system. Restoring a katra to a Vulcan is preferably done by a Vulcan priest with enough experience with katras.
Vulcans can initiate a transfer of their katra by placing one of their hands on the head of the recipient and to position their fingers on specific points. Via a mind meld, a Vulcan is able to tell if someone has received a katra.
The katra can be restored when a person has died and his Katra was transferred to someone, if the family of that person wishes to do so. This ritual is called fal-tor-pan, which literally means "the refusion". This ritual is performed very rarely. The last time was in 2285 when Spock's regenerated body was found on Genesis and returned to Mount Seleya, where his katra was transferred from Doctor McCoy into his own body.
The transfer of someone's katra is not regularly practiced and is only done in special circumstances. When a Human receives a katra, the effects of the merge can by counteracted by lexorin, but this is only for a short period of time. The katra must eventually be transferred to someone else. Around 2269 the existence of the katra was common knowledge within most medical establishments.
It is possible that katra transfers are not complete transfers but rather a copying of information, much like copying a file rather than moving it. Spock was fully functional with all his memories after the transference of his katra to Doctor McCoy which would not be possible if everything he was mentally was completely removed.