Like most home products, candles are safe as long as you use them properly. The problem is thinking that as long as a candle doesn't fall over or come into contact with anything flammable, it is okay.
However, according to the National Candle Association (candles.org) there's more you can do to ensure candle safety.
Do not place candles where they can be knocked over by pets. (Cats can be particularly fascinated by the flickering light.)
Trim the wick to 1/4 inch before burning. A long wick...