Wikipedia
Kvitel (, "little note"; plural קוויטלך, Kvitelach) refers to a practice developed by Hasidic Judaism in which a Hasid (a follower of Hasidic Judaism) writes a note with a petitionary prayer and gives it to a Rebbe (Hasidic Jewish leader) in order to receive the latter's blessing. This prayer may be a general request for health, livelihood, or success, or a specific request such as recovery from illness, the ability to bear children, a wedding match, etc.
The writing, giving and reading of a kvitel is treated very seriously by Hasid and Rebbe alike, and is executed according to specific protocols. Because of their inherent sanctity, kvitelach may not be thrown away after use; they are either burned or buried.
The practice of giving kvitelach continues today in all the Hasidic courts. Kvitelach are also placed on the graves of Rebbes and tzadikim (plural of "tzadik," or Jewish holy man) with the hope that the soul of the deceased will intercede for the petitioner in Heaven.
It is a centuries-old custom for Jews to place kvitelach containing personal prayers to God between the stones of the Western Wall in Jerusalem. This practice has been also adopted by Christian pilgrims and foreign dignitaries as well. More than a million prayer notes are placed in the Western Wall each year.