I have been asked, "what do you mean, 'anvil'?" Good question.
Here is a drawing of a blacksmith's anvil. Note it is flat and wide at the top.
Here is a photo of a thunderstorm (technically, a cumulonimbus cloud) with the anvil shape. We refer to the large flat cirrus top of the storm as the "anvil."
UPDATE: Here is a view of a thunderstorm taken from my back yard a few minutes ago. If you ignore the building, you can see an almost perfect anvil shape.