Bitcoin is controlled by a decentralized network of computers with a set of rules that are democratically agreed to by the individuals/companies that provide the computing power for said network. Each coin doesn’t exist physically, but instead sits as a balance on an enormous public ledger in the cloud that (along with all transactions) is verified by this decentralized computing power.

Notably, this makes it an alternative to central bank controlled money, although following the abolition of the gold standard, “normal” money generally has no underlying asset either.