Low Clouds
Stratus
Stratus clouds are flat and generally featureless. They don't precipitate all the time; but then they do, they produce drizzle to light rain or snow. A sky full of stratus will completely obscure the sun; but if there are breaks, you can see cool stuff like this. (Those rays of sun peeking through the cloud deck are called ‘crepuscular rays’.)
Stratocumulus
Stratocumulus look kind of like stratus; but instead of being flat, the clouds have a more rounded shape. These too only produce light precipitation.
Nimbostratus
Nimbostratus look kind of like just-plain-stratus, but produce heavier precipitation. Another way to tell the difference is to look at the cloud base. If the clouds have a defined bottom, you're looking at stratus. If the sky is just a dreary grey schmutz like the photo here, you're looking at nimbostratus.