The Sabbath lasts for 25 hours. Shabbat begins 18 minutes before sundown on Friday night with the lighting of the Shabbat candles (see the Orthodox Union website to find out the proper candle-lighting time) and Kiddush and ends an hour after sunset on Saturday night with the Havdalah celebration. There are 39 kinds of work that are forbidden on Shabbat. There are synagogue services held on Friday night and Saturday morning (some synagogues have other afternoon and evening services on Saturdays as well). [2011-2012 Torah Reading Schedule]

There are special Shabbatim throughout the year. Some have additional Torah readings (Maftir) that are read after the regular Torah readings - replacing the regular Maftir readings.

The four parshiot:
These are special Torah readings added to the regular weekly parsha in preparation for Pesach.

Shabbat Shekalim is the Shabbat before Rosh Chodesh Adar (Adar II in a leap year). The maftir reading (Exodus 30:11-16 recalls the census taken in the wilderness.
Haftarah (Ashkenazi): II Kings 12:1-17
Haftarah (Sephardi): II Kings 11:17-12:17

Shabbat Zachor is the Shabbat before Purim. The maftir reading (Deuteronomy 25:17-19) recalls the treachery of Amalekites and the command to blot out their memory.
Haftarah (Asheknazi): I Samuel 15:2-34
Haftarah (Sephardi): I Samuel 15:1-34

Shabbat Parah is the Shabbat after Purim. The maftir reading (Numbers 19:1-22) recalls the preparation for Pesach.
Haftarah (Ashkenazi): Ezekiel 36:16-38
Haftarah (Sephardi): Ezekiel 36:16-36

Shabbat HaChodesh is the Shabbat before Rosh Chodesh Nisan or Rosh Chodesh Nisan if it falls on Shabbat. The maftir reading (Exodus 12:1-20) recalls the establishment of the Jewish calendar.
Haftarah (Ashkenazi): Ezekiel 45:16-46:18
Haftarah (Sephardi): Ezekiel 45:18-46:15

Other special Shabbatim:
Shabbat Bereishit is the first Shabbat after Simchat Torah when Parsha Bereishit (Genesis 1:1-6:8) is read.

Shabbat Chazon is the Shabbat before Tish B'Av. The hafatarah reading (Isaiah 1:1-27) recalls Isaiah's vision of the destruction of the Temple.

Shabbat HaGadol is the Shabbat before Pesach. The haftarah reading (Malachi 3:4-24) recalls the promise of the return of Prophet Elijah and the ushering in of the end times.
Haftarah (Ashkenazi): Malachi 3:4-24
Haftarah (Sephardi): Malachi 3:4-24

Shabbat Mevarchim is the Shabbat before Rosh Chodesh when a special blessing (Birkat HaChodesh) is said before the beginning of the mussaf prayer.
Haftarah (Ashkenazi): I Samuel 20:18-42
Haftarah (Sephardi): I Samuel 20:18-42

Shabbat Nachamu is the Shabbat after Tish B'Av. The reading is one of the first of seven readings between Tisha B'Av and Rosh Hashannah that are designed to comfort the Jews with the promise that the Temple will be rebuilt (Lamentations 1:2).

Shabbat Shirah is the Shabbat when Parsha Beshalach (Exodus 13:17-17:16) which includes the reading of the Song at the Sea.
Haftarah (Ashkenazi): Judges 4:4-5:31
Haftarah (Sephardi): Judges 5:1-31

Shabbat Shuva is the Shabbat between Rosh Hashannah and Yom Kippur. The readings deal with repentence and G-d's mercy.
Haftarah (Asheknazi): Hosea 14:2-10, Joel 2:11-27, Micah 7:18-20
Haftarah (Sephardi): Hosea 14:2-10, Micah 7:18-20

The Sabbath candle-lighting is obligatory upon all adults (including teens who are bar mitzvah or bat mitzvah). It is traditional for the woman of the house to light the candles. By her lighting the candles and saying the brocha (blessing) all adults in the house are considered to have lit the candles. If the woman of the house is not present the man is to perform the mitzvot. If neither is at home the mitzvot falls upon a bar/bat mitzvah child (preferably a female). All single adults who live on their own (such as in a dorm) are required to light the Shabbat candles. After the candles have been lit the parents bless their children and the husband blesses his wife. The family performs the mitzvot of Kiddush with the proper brocha and has their festive Shabbat meal.