Bitcoin is an open system for transferring monetary value without a mandatory central administrator. The same name denotes the protocol, the network of participants and its monetary unit; the unit's symbol is BTC, and one BTC divides into 100 000 000 satoshis. To understand Bitcoin, it helps to follow one payment: who can control the value, how they pass that authority to a recipient and how others verify its acceptance into a shared history.
A bank balance represents the bank's obligation to its customer. With bitcoin in self-custody, we hold the means to satisfy the spending conditions of particular records in the network. The protocol records amounts and conditions, not owners' legal names. A wallet combines these records into the displayed balance; coins themselves are not files stored on a phone. A balance with a custodian or exchange means something different: it is generally a claim on the service controlling the keys. The technical ability to sign a payment does not by itself determine legal ownership. [Bitcoin Developer Guide — Transactions] [Bitcoin.org — Securing your wallet]
The recipient provides an address generated by a wallet. It instructs the sender how to create an output with particular spending conditions; it is neither a coin mailbox nor a secret password. In a common case, a digital signature created with a private key authorizes the next spend; checking the signature does not require revealing that key. Conditions can also require multiple signatures or waiting. Knowing an address is insufficient to spend. Someone who obtains the necessary secrets can nevertheless create a valid spend; the network cannot distinguish key theft from its holder's consent. [Bitcoin Developer Guide — Transactions]
An unspent output of a previous transaction is called a UTXO. An ordinary transaction references selected UTXOs through its inputs, consumes them completely and creates new outputs. Given a suitable output worth 100 000 sat, a sender can create 60 000 sat for the recipient and 39 000 sat back under their own control. The remaining 1 000 sat is the fee, not a recommended rate. The original output can no longer be spent again in that same history. The change address need not be the original address. Recipient, amount and fee are checked before signing. [Bitcoin Developer Guide — Transactions]
The wallet prepares and signs the payment; a Full Node verifies data against consensus rules. It checks, for example, that inputs exist and remain unspent, that spending conditions are met and that an ordinary transaction creates no extra value. It may accept a valid unconfirmed transaction into its local waiting area, the Mempool, and relay it to peers. Each node has its own waiting area and relay policies; refusing relay does not always mean invalidity in a block. A wallet can use its own node or an external server supplying information. Even one's own node cannot protect a stolen signing key. [Bitcoin Developer Guide — Transactions] [Bitcoin Developer Guide — Operating Modes]
An authorized key holder can sign two different transactions spending the same UTXO. Both signatures can be correct, and different participants may initially see different payments. Alongside authorization, we therefore need a shared ordering. Double spending does not mean copying a coin file: it is an attempt to establish two conflicting uses of the same output. At most one can belong to a single valid history. Message arrival time, the number of connected nodes or a wallet's claim cannot settle this conflict globally on their own. [Bitcoin: A Peer-to-Peer Electronic Cash System]
Miners assemble candidate blocks and repeatedly calculate hashes to find Proof of Work meeting the difficulty target. A block references the previous block; changing older history requires recomputing the subsequent work. A node first verifies the rules, then selects the valid branch with the most cumulative work, not necessarily the most blocks. Mining thus helps establish order; it grants no authority to spend other people's outputs. Even a majority of computing power cannot force an unchanged node to accept an invalid block, although it can disrupt ordering and the availability of confirmations. [Bitcoin Developer Guide — Block Chain]
Inclusion in an accepted block gives a transaction its first confirmation. Subsequent blocks generally increase the cost of replacing that history. Block intervals are not a timetable: ten minutes is a long-term target, not a promised payment time. A reorganization can remove a previously accepted block, returning a transaction to unconfirmed status or replacing it with a conflicting spend. The required confirmation count therefore depends on payment value and risk; it gives no absolute certainty. The protocol has no administrator for cancellations. A voluntary refund is a new payment whose fee must be addressed again. [Bitcoin: A Peer-to-Peer Electronic Cash System] [Bitcoin Developer Guide — Block Chain]
The block's first transaction, the Coinbase Transaction, may allocate the block subsidy and included transaction fees to the miner. Only the subsidy creates new units; fees transfer existing value. The maximum subsidy began at 50 BTC and halves every 210 000 blocks. This Halving does not apply to fees. Rounding to whole satoshis keeps the sum of permitted subsidies below 21 million BTC. Nodes reject rewards exceeding the permitted amount. Lost keys do not increase future issuance, and a limited number of units does not itself determine purchasing power. [Bitcoin Developer Guide — Block Chain]
Source code can be examined and changes proposed publicly. A developer releases software, a node operator chooses the rules to verify, and a miner chooses a candidate block's contents. No role alone decides for everyone; incompatible changes can split the network. Participants nevertheless have unequal economic influence. Limited block space constrains the requirements for verifying and relaying data, but high demand creates competition for inclusion. Fees therefore relate to transaction size and demand for inclusion, not simply to the amount transferred. [Bitcoin Developer Guide — Block Chain] [Bitcoin Developer Guide — Transactions]
Public history enables verification, but also links between payments and identities, for example using exchange records. Pseudonymity is not anonymity. Self-custody removes the need to ask a custodian to sign, while making the holder responsible for backups, device security and checking recipients. A backup must restore the necessary keys and configuration; restoring a wallet alone cannot make an exposed secret secret again. One's own node helps verify history but guarantees neither price, privacy nor safe signing. Independent verification means knowing which of these questions is being checked. [Bitcoin.org — Protect your privacy] [Bitcoin.org — Securing your wallet]
For the clearest picture, read this entry together with Satoshi Nakamoto, Bitcoin Whitepaper, Genesis Block, Proof of Work, Full Node, 21 Million. The reverse links also lead from Satoshi Nakamoto, Bitcoin Whitepaper, Genesis Block, Full Node.