Shark Tank is the American adaptation of the business-pitch format used by Dragons' Den.
The series premiered on ABC in August 2009 and places entrepreneurs before a panel of wealthy investors, who decide whether to offer money in exchange for equity or another financial arrangement. Its title refers to the investors as sharks rather than dragons, but the central pitch-and-negotiation structure is closely related.
Dragons' Den itself originated from the Japanese format Money Tigers, while Shark Tank was developed for the United States market. The two programmes are therefore related through the wider international format family, not because Shark Tank is a direct continuation of the British series.
A common mix-up is to call Shark Tank the original version. It is the US adaptation; the Japanese programme is the format's original source, and Dragons' Den is the name used by several national versions, including the UK and Canadian editions.