The lighting of a Unity Candle is a relatively recent addition to the traditional wedding ceremony. It is 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 symbolizes the union of two individuals, becoming one in commitment.
A Unity Sand Ceremony is a more lasting memento than the traditional candle lighting ceremony. In a sand ceremony, the bride and groom are instructed to each pour sand into a vase either one at a time or simultaneously, whilst a few words are read aloud. The sands are considered to be symbolic of the couple and their union of marriage.
You can read more information on Unity Sand & Candle Ceremonies here.