The first group consists of the Normal and Dissolve modes, which have no obvious effect.
The modes in the second group darken an image and those in the third are used to perform general image lightening.
The fourth group makes dark tonal values even darker while brightening lighter ones, and the fifth group creates a mathematical relationship between the two basic tonal areas.
The sixth and final group combines tonal values, saturation, color and luminosity in a variety of ways.
--Developing Images Using Blend Modes and Layer Techniques, Michael Jordan
An excellent discussion titled "Blending Modes for Photographers" can be found at Earthbound Light