The difference between the radiation budget components for average cloud conditions and cloud-free conditions. Roughly speaking, clouds increase the albedo from 15 to 30%, which results in a reduction of absorbed solar radiation of about 50 W/m^2. This cooling is offset somewhat by the greenhouse effect of clouds which reduces the OLR by about 30 W/m^2, so the net cloud forcing of the radiation budget is a loss of about 20 W/m^2. Were the clouds to be removed with all else remaining the same, the Earth would gain this last amount in net radiation and begin to warm up.
Source: NASA (http://earthobservatory.nasa.gov/Glossary)