The Evening Shabbat Prayer - kabolas shabbos
During Kabbalat Shabbat - when we welcome the Shabbat - we say six psalms from Tehillim, from Psalm 95 through Psalm 100, followed by Psalm 29. These psalms are called "Lecho Neranena" after the first words of the opening psalm.
After reciting these psalms, we say the prayer "Lecha Dodi," composed by Rabbi Shlomo Alkabetz - a great Kabbalist who lived in the city of Tzfat during the time of the Arizal. After this prayer, we say the Song of the Day for Friday and Shabbat.
The six psalms of Kabbalat Shabbat correspond to the six days of creation, and the psalm said after Lecha Dodi, "Mizmor Shir L'Yom HaShabbat," corresponds to Shabbat itself.
Following this, we say "Kegavna" - a passage from the Zohar describing the elevation of Shabbat evening, when the attribute of Malchut separates from the Sitra Achra (other side).
Although the Ramak established the recitation of the six Lecho Neranena psalms, and the Ari - who knew of this as he lived during the same period - did not include these psalms in his siddur, the Alter Rebbe chose to include the six psalms in his siddur, following the Ramak's opinion.
Time of Prayer
Although generally one should pray the evening prayer at nightfall, on Shabbat it is permissible to pray Maariv starting from Plag HaMincha (1.25 halachic hours before sunset), since some consider Plag HaMincha as nighttime. However, one should be careful not to pray Mincha after Plag HaMincha in such a case. The reason for praying early is to add from the weekday to the holy. Nevertheless, one must be careful to recite the Shema at nightfall. However, the Chabad custom in practice is to delay the Shabbat evening prayer, and one should study Chassidut before prayer.
Order of Prayer
We begin the prayer with Borchu, followed by the blessings "Maariv Aravim" and "Ahavat Olam," then we recite the Shema. We continue with the blessings after Shema, and before praying the Amidah, we say "Hashkiveinu." Unlike other prayers, we don't conclude with "Shomer et amo Yisrael la'ad" because Shabbat guards us, and instead conclude with "Blessed are You, Hashem, who spreads the shelter of peace, etc."
Some communities have the custom to say the verse "V'shamru" before the Amidah to express that Shabbat guards us, and that if Israel keeps two Shabbats they will be redeemed. However, many communities, including Chabad, do not say this passage as it constitutes an interruption. It is told that Rabbi Levi Yitzchak of Berditchev asked the Alter Rebbe why he omitted "V'shamru," saying that when it is recited, a great fair of angels takes place. The Alter Rebbe responded, "We don't need to participate in every fair."
The Shabbat evening Amidah contains seven blessings: three opening blessings, three regarding Shabbat, and one concluding blessing. After the prayer, we recite Vayechulu, which should be said together aloud while standing, even when praying alone. We then say the blessing "Me'ein Sheva" which the Sages instituted because people used to pray in fields, and to prevent those who extended their prayers from remaining alone in the fields, they lengthened the prayer. It should be said with a minyan, though an individual may say it and should not be prevented from doing so. Afterward, we say half-Kaddish, then the psalm "Mizmor L'David," another half-Kaddish, then Aleinu L'shabeach, and before saying Aleinu, we say Borchu.
For Further Reading
Shulchan Aruch HaRav, Chapter 261