Nimbostratus

Height: These clouds are very low, they are found below 2000 meters.

Shape: Mean looking clouds.

Color: These are dark rain clouds.

Weather: Rain or snow is often when there is nimbostratus clouds.

Source for picture: http://www.uwsp.edu/geo/faculty/ritter/images/atmosphere/clouds/nimbostratus_NOAA_wea02042.jpg