Some ritually slaughter a chicken before Yom Kippur for “kaparot” (“atonements”). The idea is that whatever was supposed to befall the person should befall the chicken instead, sparing the person. (A rooster is a “gever” in Hebrew, the same as the word for “man.”) Some say that we should not perform this ceremony, as it is considered to be “darkhei emori,” a gentile custom based on foreign beliefs, with no Torah source. Rashba writes that when he first arrived in his city, he found that kaparot was widely practiced, along with other strange customs which the Jews had learned from the non-Jews. He explained to the community that this custom was disgraceful and ordered them to stop it (Responsa Rashba 1:395). This is also the ruling of Shulḥan Arukh (605:1).
In contrast, Rema writes that the custom should not be abandoned, as it is a venerable one, dating back to Geonic times. This is also the position of Arizal. Due to his influence, many people, both Ashkenazim and Sephardim, perform kaparot. This is how it is done: On Erev Yom Kippur, in the early morning, a chicken is acquired for each member of the household. It is passed over their heads, and they say, “Let this be my exchange, let this be my substitute, let this be my atonement, etc.” The chicken is then slaughtered. When possible, a rooster should be acquired for each male, and a hen for each female. A pregnant woman should use a hen for herself, and both a rooster and a hen for her fetus. If white chickens are available, they should be used, to recall the verse, “Be your sins like crimson, they can turn snow-white” (Yeshayahu 1:18). While the chicken is being slaughtered, the person should have in mind that the chicken is his stand-in; everything happening to the chicken really should be happening to him. Afterward the custom is to give the meat or its monetary equivalent to the poor, so that they will have food for proper meals before and after the fast. Someone who cannot obtain chickens can fulfill the custom using geese or fish. There are only two requirements: that the animals are kosher, and that they cannot be offered as sacrifices (so that it does not look as if one is offering a sacrifice outside the Temple precincts).
Over the course of time, as the custom became more widespread, ritual slaughterers found themselves under a lot of pressure to work very quickly on Erev Yom Kippur. This led to concern that they might not check their knives properly or slaughter correctly. As a result, poskim ruled that it was preferable either to delay the slaughter until after morning services or to have it done a day or two early, since any time within the Ten Days of Repentance is an appropriate time for the ritual.
Nowadays, many people fulfill this custom by giving charity – the value of a chicken on behalf of each family member. Those who do so because of the difficulty of ensuring proper slaughter pass the money overhead instead of a chicken. Those who do so because they feel that this ritual is non-Jewish in origin do not pass the money over their heads. They simply give it to charity, which is certainly an appropriate thing to do before Yom Kippur.
Each person can choose which custom to follow. Nevertheless, if his family has a custom, it is preferable to follow it.