Though Kelvins are generally a measurement of heat, when talking lights, Kelvins are about the color of light: the lower the Kelvins, the more red spectrum you get; the higher the Kelvins, the more blue spectrum you get. In short, the hotter the fire (more Kelvins) the bluer the flame, and vice versa. This is why the bottom of the flame on your bic is blue -- it's hotter than the orange at the tip. Anyhow, as others have said, the lower K/red spectrum are better for flowering as they replicate the sunlight in the fall.
Watts is the measurement of the amount of energy that the bulbs are turning into light and heat. The more energy you put into your system/grow box, the more energy your plants will have for photosynthesis (of course, there is such thing as too much, but that's quite a bit more than any of us will ever practically use, particularly with CFLs).
Lumens is a measurement of the apparent brightness of lights, and based on the apparent brightness of a "standard candle" to harken back to high school physics. There seems to be some dispute about the use of lumens in rating grow lights, as it is based on the apparent brightness, which can vary from person to person. Nevertheless, it is a commonly used rating, and seems to impact upon light penetration (but this is more my conjecture than fact). To put it in perspective, sunlight is rated at approximately 10,000 lumens per square foot. To replicate that with CFLs, you'd need about 6 or 7 at least.
Of course, the good rule of thumb with CFLs seems to be: the more you use, the better. Happy Growing!