Candle magic is simply the incorppration of candles in spell casting. Some people choose to use a candle as a focal point, or a symbolic representation of different aspects of what they are trying to accomplish.
The flame may be used as a representation of energy, or cleansing, purging, or whatever the person wants it to be.
The color may be used specifically for some correspondence the person wants to convey.
Burn time, the way the wax melts, dripping the wax onto objects or into containers, uses for divination, and so much more, are all incorporated into candle magic.