Because of the two different phrases (Zachor and Shamor) in which the Torah tells us to observe the Sabbath (see Exodus ch.20 and Deuteronomy ch.5). We light two candles to remember these commandments throughout the sabbath day.
The Shabbat is really supposed to start at sundown. But Jewish custom and obligation is to welcome Shabbat a few minuets before it would normally start. This can be done in several ways (such as saying "I welcome Shabbat").
Regardless to this, there is another obligation - which is to light before the beginning of Shabbat candles, so there will be light in the house on Shabbat. (One is forbidden to light fire on Shabbat).
The common Jewish custom to light Shabbat candles on the beginning of shabbat is in fact a combination of these two obligations because the lighting of the candles is one of the ways to welcome the Shabbat a few minuets before sunset.
For the reason of lighting two candles please see related question. Please notice that many Jews light more then two candles, sometimes as many as the family members.
To signify the two biblical expressions regarding the commandment of the Sabbath, namely "zochor" and "shomor" ("remember" and "guard" respectively).
We light one candle to remember Shabbat and one candle to observe Shabbat (which are the two different ways that the 4th Commandment is worded).
Traditionally, the Shabbat candles are lit by women. If the woman is unmarried, she will usually light one candle, if she's married two and often one for each child.
They're called Shabbat candles (Neirot Shabbat in Hebrew).
You don't light more candles than usual. In the blessing over the candles, the Shabbat ismentioned first.In the event that a more frequent occasion coincides with one less frequent, the morefrequent is acknowledged and blessed first.
If you are talking about a Hanukkah menorah (also called a Chanukiah), then you would light the Chanukiah first, and say the 2 hanukkah blessings (3 if it's the first night). Then you would light the Shabbat candles and say the blessing over the shabbat candles.
They're called Shabbat candles (in English); neirot Shabbat (in Hebrew); or Shabbes Licht (in Yiddish).
They prepare their children for Shabbat and light the Shabbat candles.
Traditionally, the Shabbat candles are lit by women. If the woman is unmarried, she will usually light one candle, if she's married two and often one for each child.
The Shabbat is observed by all Jews, both men and women. However, it is the women who traditionally light the Shabbat-candles. See also the Related Links.Link: Why do women light the Shabbat candles?Link: Women and the lighting of the candles
The Shabbat is observed by all Jews, both men and women. However, it is the women who traditionally light the Shabbat-candles.See also:Women and the lighting of the candles
They're called Shabbat candles (Neirot Shabbat in Hebrew).
At a shabbat service at your home or at a synogauge you may light the shabbat candles and say the shabbat blessings. Also some people follow it with hallah and/or a kiddush.
They symbolise light. They mark the presence of the Shabbat itself, and also hint to the Divine presence.
The Shabbat is observed by all Jews, both men and women. However, it is the women who traditionally light the Shabbat-candles.
You light 2 on the first night. Then each night, you add another candle, ending with 9 on the eighth night.
You don't light more candles than usual. In the blessing over the candles, the Shabbat ismentioned first.In the event that a more frequent occasion coincides with one less frequent, the morefrequent is acknowledged and blessed first.
If you are talking about a Hanukkah menorah (also called a Chanukiah), then you would light the Chanukiah first, and say the 2 hanukkah blessings (3 if it's the first night). Then you would light the Shabbat candles and say the blessing over the shabbat candles.
No Orthodox or conservative Jews should put 13 candles in their window. The most candles they will put in the window is 11: Eight for chanukkah, 1 shamash and 2 shabbat candles. But Shabbat candles are not normally placed in windows. In some orthodox homes young girls light candles to copy their mothers (who are also lighting candles). This might explain why there could be 13 or more; but for Jewish purposes only 11 are necessary. The rest are extra.