Because compact fluorescent bulbs use less electricity, the wattage ratings are different. a 60w replacement usually are 11-15 Watts!
The cool thing is that because of the difference, you can get say a 100 Watt CF Bulb and it will only use 24 Watts - but provide more light than a standard 60!
You shouldn't use compact fluorescent bulbs with dimmer switches, unless the bulb is specifically marketed as compatible. Also, beware of the old lamps where the shade actually rested on the bulb..
Most will actually list actual wattage and what it replaces.
Assuming you are in the US, a 13W bulb replaces a 60W standard bulb.