What type of volcano is Krakatoa?

Krakatoa is a Caldera Volcano.

A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot." In some texts the English term cauldron is also used.

In 1815, the German geologist Leopold von Buch visited the Las CaƱadas caldera of Teide on Tenerife, and the Caldera de Taburiente on La Palma, both in the Canary Islands. When he published his memoirs he introduced the term caldera into the geological vocabulary.