A traditional vertical clay oven, fired by charcoal. Food such as meat is cooked on skewers over the coals; bread can be cooked by slapping it against the walls. It says here. This is a big part of the reason why naan is flat.

Modern not-quite-as-traditional ones made of steel are to be had.

Associated with Indian food, but actually a common Middle Eastern word and thing: the English tandoor represents Urdu and Punjabi tandûr. The D is extraneous, apparently arising in Turkish. Older cultures in the region lack it: Arabic, Hebrew, and Persian tannûr, Aramaic tannûrâ, Assyrian tinûru. The spellings tanoor and tanur are sometimes seen for the Arab and Jewish clay oven.

Food cooked in a tandoor is called tandoori. The temperature is very high so cooking is fast.