Shabbat is the name of the day of rest in Judaism. Shabbat happens on the seventh day (Saturday) of every week.  In Judaism, the day is defined with the cycle of the sun: The day begins and ends at sunset, not midnight.  So the seventh day of the week, Shabbat, begins Friday when the sun goes down, and ends Saturday night after it gets dark.  The idea of Shabbat comes from the Bible's story of Creation. In that story, God creates the Universe and everything on Earth for six days. On the seventh day, he stops work. In the same way, Jews work on the first six days of the week and rest on the seventh day, Shabbat.

The word Shabbat began as a Hebrew word (). The English word "Sabbath" comes from the word "Shabbat". The English word can also be used to refer to Shabbat. The Christian idea of Sabbath came from the Jewish idea of Shabbat. Now, there are many differences between them.

The first part of the Jewish Bible, the Torah, says that work is not allowed on Shabbat. It is a day to rest and study the Torah.

In Jewish law, Shabbat is the most important Jewish holiday. It is even more important than Passover, Rosh Hashanah and Yom Kippur.  Jewish mystics say that the Shabbat day is meant to be like a perfect world. In this world, everyone knows about God and loves him.  They believe that kind of world has not been seen since the Garden of Eden, and will not be seen again until the Messiah comes.

Two parts of Shabbat: celebrating and guarding
There are two versions of the Ten Commandments in the Bible. These versions use different words.  The version in Exodus says "Remember Shabbat to keep it holy."  The version in Deuteronomy says "Guard Shabbat to keep it holy."  Jewish custom says that "remember" means to celebrate Shabbat.  "Guard" means resting—not working or doing business.

Celebrating Shabbat

Shabbat is a happy day. There are many ceremonies of Shabbat that help people celebrate the happiness of the day.

Shabbat candles.  It is a Jewish law that no one may light a fire after Shabbat starts.  Because of this, someone in every home lights candles just before Shabbat.  The woman of the house  usually lights the candles, but not always.  The candles should last until the people in the house finish dinner.  By lighting candles when Shabbat begins, Jews make certain they will have lights in the home when they celebrate.  This adds to the joy of the celebration.  Having candles on Shabbat is a very old Jewish custom.
Blessings on wine.  A verse in the Bible's Book of Psalms says, "Wine makes the heart of a person happy." Because of this verse, Jews usually celebrate happy occasions with wine.  The two main Shabbat meals start with a blessing over a cup of wine.  Most of the time, the cup that people use for this blessing—called Kiddush—is fancy. (See picture at right.)
Three Shabbat meals.  On every Shabbat, Jews have three meals.  The first is at night, after the Friday night prayer service.  The second is at noon, after the Saturday morning prayer service.  The third is late Saturday afternoon, just before Shabbat ends.
Each of the first two meals begins with a blessing over wine.  Next, there is a blessing on bread.  In the Bible, God gave the Israelites two portions of manna every Friday so they would not need to collect it on Shabbat. At each of the first two Shabbat meals, there are two loaves of bread. This is to remind them of the double portion of manna. At these meals, Jews serve the best food they have money to buy.  They use their best plates and silverware. At each meal, they sing special songs, called zemirot, to honor Shabbat.  The first two Shabbat meals are large and formal.  
The third Shabbat meal is often smaller and less formal.  Some Jews include bread in this meal, while others do not.  Many people call this meal Shalosh Seudot ("three meals") because eating this meal completes the full set of three meals for Shabbat.  This meal is often accompanied by zemirot and Torah study.
Shabbat prayer services.  Shabbat prayer services are organized much like weekday prayer services.  There are some changes from the weekday order.  The most noticeable changes are:
A special service, Kabbalat Shabbat ("Receiving Shabbat") is added between Friday's afternoon and evening prayers.
The Amidah prayer is changed on Shabbat.  During the week, the Amidah includes thirteen blessings asking for God's help with everyday life.  On Shabbat, those are replaced by a single blessing thanking God for the rest day of Shabbat.
The set of Psalms which is read at the start of the morning prayers is made longer on Shabbat.
The entire weekly Torah portion is read out loud from a handwritten parchment scroll.
An additional Amidah, called Musaf ("additional service"), is added near the end of morning prayers.  This prayer is said in place of the additional offeringdef. n2 that was brought to the Temple in Jerusalem each Shabbat.
During the Saturday afternoon prayers, the beginning of the Torah portion for the next week is read out loud.
Other kinds of celebrating.  Oneg Shabbat, which means "enjoying Shabbat", is an important part of the day.  Some of the ways Jews enjoy Shabbat were already  described above.  Other ways include wearing nice clothing, socializing with friends and family, physically resting or sleeping, studying Torah and other religious books, and marital relations (sex between a husband and wife).

Guarding Shabbat—rest and work on Shabbat

Enjoying Shabbat is very important, but Shabbat is a holy day.  There are laws and rules in the Torah, the Mishnah and the Talmud about what Jews may and may not do on Shabbat.  These laws help Jews keep Shabbat as a holy day.
39 prohibited categories of work.  The Mishnah lists 39 categories of work (Hebrew: melachah) that Jews should not do on Shabbat.  The laws about what actions are in each category are complicated.  All the categories are about making new things or repairing or cleaning old things to make them useful again.

Here is a partial list of work categories not allowed on Shabbat:
Lighting a fire 
Carrying anything outdoors in a public space
Cooking
Cleaning, except some washing for hygiene (like washing your hands before eating, or cleaning a fresh cut in the skin)
Traveling more than about 3 km (2 miles) from your city or home
Writing
Farming work
Sewing and weaving
Building or tearing down
Preventions. A prevention (Hebrew: shevut) is an action that does not fall into one of the 39 categories of work, but is still prohibited.  Rabbis prohibited these activities so that Jews would not do work in one of the 39 categories (even by accident or carelessness).  
Buying and selling things with money is a prohibited prevention. It is not in of the 39 categories of work.  But someone who buys or sells things might do something in the 39 categories of work because of it.  For example, she might write a receipt or carry what she buys in a public space.
Electricity on Shabbat.  Useful electricity is much newer than the laws of Shabbat.  Rabbis have worked hard to try to understand how electricity fits into the laws of Shabbat.  They have written whole books on the subject.  Most Orthodox Jewish rabbis—but not all—have decided that:
Using electricity to make something very hot (like an oven or a light bulb) is the same as lighting a fire or cooking.  Using electricity this way is in the 39 categories of prohibited work.
Using electricity to do anything in the 39 categories of prohibited work is not allowed.  
Using electricity for most other things is a prohibited prevention.  This includes talking on a telephone or using a computer.  
Some electrical devices (like lights) can remain on if started before Shabbat. 
Saving a human life.  Jews learn from  that one must violate the laws of Shabbat to save a human life.  Sometimes, there are two different ways to save a life.  Then the person should try to pick the way that violates Shabbat least.  But that is not always possible.  And saving a life always comes first.  Even if someone is not sure he will be successful to save a life, trying to save life on Shabbat always comes first.

Shabbat in non-Orthodox Judaism

Conservative Judaism also teaches Jews not to do activities in the 39 prohibited categories of work (melacha). Conservative rabbis are often less strict about what is melacha than Orthodox rabbis are.  Conservative rabbis are almost always less strict about what is a prevention (shevut) than Orthodox rabbis.  For example, Conservative rabbis allow Jews to use electricity on Shabbat for many purposes.  They do not allow Jews to use electricity to do any of the 39 prohibited categories of work.

Many, if not most, lay members (not rabbis or prayer leaders) of Conservative synagogues in North America do not follow these laws.  Progressive Judaism, including Reform Judaism, does not accept Jewish law as binding. These Jews may rest on Shabbat, but are not usually strict about not doing melacha or shevut. They may even add practices not allowed under Orthodox Jewish law that they think improves their celebration of Shabbat. For example, they may drive to synagogue on Shabbat, or may use musical instruments on Shabbat. Orthodox Judaism does not allow either of those actions on Shabbat.

Havdalah: Ending Shabbat

Shabbat ends after dark on Saturday night. The end of Shabbat is marked by a ceremony called Havdalah ().  This is a Hebrew word meaning "division" or "separation".  The ceremony "divides" or "separates" the holy day of Shabbat from the new week.  The Havdalah blessings are spoken over a cup of wine.  The Havdalah ceremony also uses spices with a nice smell and a candle with many wicks (cloth strings for lighting).  After Havdalah, people start doing regular weekday things again.

Sources
Note 1. Sources here may not be in Simple English.
Note 2. Some Bible sources below include a Simple English version of the Jewish Publication Society's 1917 English translation.  All Bible sources below include a link to the 1917 translation and the original Hebrew text.

Other websites

Jewish festivals and holidays