The second chapter of Tractate Shabbat has an extensive discussion about what materials may be used for lighting Shabbat candles. The general principle is that the wick needs to be soft and absorbent so that it will draw the oil nicely, and the oil must be of a type that is drawn nicely into the wick. If these conditions are not met, the flame will jump around the wick and the light will dim. There is concern that one seeing such light will try to fix it, thus transgressing the prohibition of lighting a fire on Shabbat.
The Sages said that of all of the acceptable oils, olive oil is the best because it is easily drawn into the wick and its light is clear and pure (SA 264:6). Today most women light paraffin or “wax” candles, whose light is stronger and more stable.
The extensive discussion about what materials may be used to light candles alludes to the way to achieve shalom bayit. Just as the wick has to be made of a soft and absorbent material and the oil must be light and easily drawn, so too a couple needs to unite humbly. Just as when we light the wick, fire and light are produced jointly by the wick and fuel, so too a couple, through the fire of faith and light of Torah, can become one, developing and illuminating together. Without a spiritual destination love withers, just as everything physical atrophies. But when there is a shared spiritual goal that lights the fire of their lives, their love becomes stronger and stronger.
The law requires that only one candle be lit, thus providing light in the home. However the custom is to light two candles, one corresponding to Zakhor and the other to Shamor. Some women customarily light one candle for each member of the household; others light seven candles, corresponding to the seven days of the week; still others light ten, corresponding to the Ten Commandments. All these customs apply when a woman is in her own home, but if she is a visitor in someone else’s home, the custom is that she lights just two candles (SSK 43:3).
In the past, when homes were normally candlelit, adding more candles beautified Shabbat by increasing light in the home. Now that electric light is common, adding candles does not increase beauty, and it is sufficient to light just the two candles corresponding to Zakhor and Shamor.
If a woman forgets to light candles one week, it is customary for her to add an extra candle every week as a penalty (Rema 263:1). However, this only applies if there was no light at all as a result of her omission. If there was electric light, even though she did not light it to honor Shabbat, since in fact they did not miss out on oneg Shabbat, she is not obligated to add a candle every week for the rest of her life (See Darkhei Moshe, ad loc.; BHL s.v. “she-shakheĥa”; Yalkut Yosef 263, n. 42).