Bitcoin is a network for transferring a single asset, and the asset exists only as balances on that network's ledger. It launched in January 2009 with software published by a person or group using the name Satoshi Nakamoto, whose identity has never been established. Nothing about how the network runs depends on knowing it.
The rules that define the asset are unusually simple and unusually rigid. New bitcoin is created only as a reward paid to whoever produces a valid block. Blocks are targeted to arrive roughly every ten minutes, with the network automatically adjusting how hard they are to produce as more or less computing power joins. Every 210,000 blocks — about four years — the block reward halves. The series that produces converges to a cap of twenty-one million, and the last fraction of a coin is not scheduled to be issued until well into the next century.
None of that is enforced by a promise. It is enforced by the fact that every node independently checks every block against the same rules and rejects one that pays its producer too much. Changing the cap would require persuading the overwhelming majority of node operators, miners, exchanges and holders to run different software simultaneously — and each of them has an obvious financial reason to refuse. This is what people mean when they call the supply credible rather than merely stated.
The consensus mechanism is proof of work. Miners compete to find a value that makes a block's hash fall below a target, which can only be done by trying enormous numbers of candidates. The winner takes the block reward plus the fees attached to the transactions included. The energy this consumes is the security budget: rewriting history means out-computing everyone else, and the cost of that is deliberately not small. Whether the trade is worth making is a genuine argument, and it is an argument about values rather than about mechanism.
The design's costs are as structural as its guarantees. Throughput is limited by block size and interval, so the base layer settles a modest number of transactions per second and fees rise when demand for block space does. The scripting language is deliberately restricted, so complex applications are built elsewhere. And as the block reward halves towards nothing, the network's security will have to be paid for by transaction fees alone — a transition that is decades away and has never been tested.
Two frequent misunderstandings are worth clearing. Bitcoin is pseudonymous, not anonymous: every transaction is public forever, and analysis firms are effective at linking addresses to identities, particularly where a regulated exchange sits at one end. And there is no such thing as a bitcoin account with a provider behind it. When a custodian holds coins for a customer, the customer holds a claim on that custodian, with all the counterparty risk that implies — a distinction that several bankruptcies have made expensively clear.