The prayer Kol Nidre, which translates as "all vows", oaths or promises, is repeated three times during the evening prayers which begin Yom Kippur. Because of the tight association of this prayer with the services on the evening (Erev) of Yom Kippur, the services are sometimes called Kol Nidre services.
One of the reasons why the Yom Kippur Evening prayer is often called Kol Nidre Service is that it is so very different from the typical night services. It is the only time when men wear Talitoth (Taleisim) and when a lengthy Selichoth service is added, followed by Avinu Malkenu. Rabbi Joseph B. Soloveitchik (z'l) even went as far to say that it would be a misnomer to call this service Maariv, and that Kol Nidrei Service (aka Kolnidre Gebet) is the proper name for it.
To come: more about the history and reasons for this prayer.