The unity candle ceremony uses two taper candles with a large pillar candle (called the "unity candle") in the center. At the beginning of the wedding ceremony, a representative from each family (usually the mothers of the bride and groom) light the two taper candles. Later in the ceremony (usually after the formal vows), the bride and groom use the two taper candles to light the large pillar (unity) candle together.
Often a unity candle is decorated with the wedding invitation, an inscription, a picture of the couple, or other ornamentation. The candles are almost always white. The lighting ceremony may be accompanied by special music, an explanation of the symbolism, or just some period of mutual gazing by the happy couple. In some circles, it is customary for the couple to save the unity candle and relight it on anniversaries.
It is sometimes performed to symbolize the joining together of the two families, and their love for the bride and the groom, into one united family that loves the new husband and wife. More often it is to symbolize the union of two individuals, becoming one in commitment. The popular explanation is that the taper candles are lit by representatives from each family to symbolize the love and allegiance that each family has for either the bride or the groom. As the bride and groom use these two flames to light the unity candle, they bring the love of both families together in a united love of the new couple. Generally, the two tapers are left burning and replaced in their holders (because each family's love for their own will continue). However, in some ceremonies they may blow out their individual candles.
Here is a video of a unity candle at a wedding: