Baptism for the dead
Baptism for the dead is the ritual of baptising deceased persons by proxy, currently practiced by Mormons. Mormons do not baptise corpses. Instead they are baptised on the deceased's behalf.
Mormons believe that certain rituals, including immersive baptism by someone holding the Aaronic priesthood, are necessary for salvation. Because billions of people have lived and died before Mormonism was founded, without having the opportunity to be converted to Mormonism, Joseph Smith introduced the doctrine of baptism for the dead. Mormons perform geneological research to find the names of their ancestors, on whose behalf they are baptised.
Criticism from other religions
Mormons have been criticized by many other sects, who find it offensive that Mormons baptise (by proxy) persons who are also their ancestors.