CUMULONIMBUS INCUS at Mykonos, Greece - photo by Sfortis, via Wikipedia; a cumulonimbus cloud which has formed the characteristic flat, anvil-top shape. They can form into supercells resulting in severe storm phenomena such as tornadoes.
Virga clouds look a bit like jellyfish floating in the sky. The "tentacles", which you can see hanging down, are called "virga". They are essentially rain, or snow, that evaporates away before ever reaching the ground. Virga common over Arizona