The "Unity Candle" is one of the most common rituals done at weddings. It's done with at least two thin tapered candles and one larger center candle, also called a 'unity' candle. It is aptly named because it symbolizes the unity of the couple; the merging of their two lives.
This can be done a few different ways. The simplest way is that the minister lights the two tapered candles before the ceremony. When the ceremony calls for it, the minister moves aside, the couple steps up and each takes a tapered candle, lighting the center one together. Note: This is not recommended for outdoor weddings.