Stratus:  Low, gray cloud layer or sheet with a fairly uniform base; sometimes appears in ragged patches; usually does not produce precipitation, but occasionally may give drizzle or snow grains; often forms by evaporation or lifting lower layers of a fog bank; composed of minute water droplets, or if the temperature is low enough, partly of ice crystals; usually widespread horizontally. 



Stratocumulus:  Gray or bluish patches or layers with individual rolls or rounded masses; generally composed of small water droplets.  If you hold your hand at arm's length, the individual cloud elements will appear to be about the size of your fist.



Nimbostratus:  A gray or dark massive cloud layer, diffused by more or less continuous rain, snow or sleet; precipitation usually reaches the ground; composed of suspended water droplets, sometimes supercooled, and of falling raindrops and/or snowflakes.  The fragmented cloud beneath the nimbostratus is Stratus Fractus or Scud.



Cumulus     Middle     High