Also known as steel drums or pans, they have a long history that goes back to freed slaves descendants from Trinidad and Tobago. After riots on the islands in the 1880s, African percussions were banned, leading to their replacement with bamboo sticks, which were later also banned. Percussions being an important part of the islands culture, they kept evolving over the years.
In the 1930s, percussion bands were using domestic items such as spoons and gin bottles, frying pans or dustbins. Steelpans appeared a bit later, made from 55 gallon industrial drums (hence the "steel drum" name referring to the container). They were popularized around the world by the United States Navy who arrived on Trinidad in 1941.
In fact, steelpans are not drums because they don't belong to the membranophones family. They are idiophones, meaning they create sound by the whole instrument vibrating.
Steelpans have a concave shape and are played on the "inside" with mallets. The instrument is tuned by carefully hammering its different parts, and notes are produced depending on which part the mallet hit.