Caldera ("cal-dare-uh") is a Spanish word meaning "cauldron", and it describes a type of large, bowl-shaped volcanic structure. Calderas are created by collapse of the roof of a magma chamber after the chamber's contents have been removed, either in effusive or explosive eruptions. They're technically not craters, which are smaller and usually located on the summit of a volcano, but much larger features that form when a volcano expels the contents of a big magma reservoir and then collapses. (If activity continues after this happens, it can even create new stratovolcanoes within the caldera.)
The eruptions that form calderas are big - hundreds to thousands of cubic kilometers of material is involved. Collapse features that form over magma chambers that large are often not recognized as volcanic features until they're seen from the air, because they're simply too large to distinguish from the ground. On the volcanic explosivity index (VEI), caldera-forming eruptions top the chart - and in the case of some, are too big to even show on the chart:
To give you an idea of what this translates to in reality, here's part of the Valle Grande in the Valles caldera:
This meadow is only a fraction of the whole caldera, and you can't even see the far walls because they're hidden behind the resurgent and smaller domes. Here's a map of the whole caldera:
The eruptions that form calderas are big - hundreds to thousands of cubic kilometers of material is involved. Collapse features that form over magma chambers that large are often not recognized as volcanic features until they're seen from the air, because they're simply too large to distinguish from the ground. On the volcanic explosivity index (VEI), caldera-forming eruptions top the chart - and in the case of some, are too big to even show on the chart:
VEI figure from the USGS Volcano Hazards Program Photo Glossary.
To give you an idea of what this translates to in reality, here's part of the Valle Grande in the Valles caldera:
This meadow is only a fraction of the whole caldera, and you can't even see the far walls because they're hidden behind the resurgent and smaller domes. Here's a map of the whole caldera:
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The Valle Grande is the light green patch in the southeast; the hills in the background of the photo are the Redondo Peak resurgent dome and smaller lava domes, which are pretty common post-caldera-eruption features. Resurgent domes are thought to be related to rebounding of the caldera floor, possibly due to new magma intrusion; the lava domes represent later eruptions through fractures. (If you want to know more about the specific geologic history of the Valles Caldera, Garry Hayes over at Geotripper has a great post from last year.) Some well-known examples of calderas in the United States are Yellowstone in Wyoming, Crater Lake in Oregon, Long Valley in California, but there are plenty of others: Krakatau and Tambora in Indonesia, Santorini in Greece, and Colli Albani and Campi Flegrei in Italy, to name a few.
I'll leave you with a photo from the rim of the Colli Albani caldera in Italy, with the Faete stratovolcano to the left of center:
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