Lumens is a good estimate of light output, but for the purpose of growing plants, what you're really interested in is a value called PAR, photosynthetically available (active?) radiation. Lumens measure light that people see - PAR measures the light that plants use for energy. Unfortunately, many bulbs are not rated in terms of PAR, so often lumens are your best guide to brightness. Because the spectrum we see in is different than the spectrum that plants photosynthesize, you need to pay attention to the color of light emitted by your bulbs, to make sure that your plants are getting light that they can actually use. This is measured by the color temperature of the bulb, which is usually a number in the thousands measured in degrees Kelvin, like 6500°K. I'm not sure exactly where the Kelvin scale comes from; it's some physics thing - I think it has to do with the color of light emitted by some sort of metal when you heat it to a given temperature or something. It doesn't really matter for our purposes - all you need to know is that you can use the color temperature rating to help match your bulbs to the colors of light that plants use, and try to match the intensity of the sun in those colors.
~Joe