Weekdays have three services. Shabbat and Yomtovim have four. Yom Kippur is the only day of the year with five. The extra service is Neilah, prayed at the end of Yom Kippur as the sun is setting. Neilah means closing.
There are two views in the Talmud as to what is or was closing at this time. The first is that it refers to the Temple gates that were closed at the end of the day.
The second is that it is the gates of heaven that are closing as daylight fades (Yerushalmi Berachot 4:1). "Open the gates," our liturgy says, "For day is nearly past."
In the spirit of the seond view, the Neilah prayers are said with particular passion and urgency. They are the last opportunity to avail ourselves of the special grace of Yom Kippur.
The overriding theme of Neilah is God's readiness always to accept and forgive all who turn in repentance.
Neilah is often led by the rabbi or the most admired and inspirational member of the community (even if they have an indifferent singing voice!). The ark remains open throughout, and people stand for the whole service, if they are physically able to do so. Neilah ends with a shofar blast, the final note of the Days of Awe.