What is bitcoin? How the first cryptocurrency actually works

Bitcoin is a digital currency that lets people send value directly to each other over the internet without a bank or payment company in the middle. Instead of a central authority, a global network of computers keeps a shared public record of every transaction, called the blockchain, and new coins are created on a fixed, predictable schedule capped at 21 million. Here’s how it works, why it’s built the way it is, and what risks come with owning it.

This is general education, not personal financial advice.

Where bitcoin came from

On October 31, 2008, in the middle of the global financial crisis, someone using the name Satoshi Nakamoto published a nine-page paper titled “Bitcoin: A Peer-to-Peer Electronic Cash System.” It proposed a way to make online payments without trusting a financial institution, by solving the “double-spending” problem, the risk that a digital token could be copied and spent twice, through a public, timestamped ledger maintained by the network itself.

The network went live in January 2009, when Nakamoto mined the first block, known as the genesis block. Nakamoto’s identity has never been confirmed, and they stopped participating publicly within a couple of years. Bitcoin today is maintained as open-source software, and anyone can run it.

The blockchain: a shared public ledger

A blockchain is a list of transaction records grouped into blocks, with each block cryptographically linked to the one before it. NIST’s Blockchain Technology Overview describes it as a tamper-evident and tamper-resistant digital ledger distributed across many participants.

Here’s what that means in practice:

• Every full node keeps a copy. Thousands of independently run computers store the entire history and check every new transaction against the rules.
• Blocks are chained by hashes. A hash is a short digital fingerprint of data. Each block includes the hash of the previous block, so altering an old transaction would change its fingerprint and break every link after it.
• Records are public. Anyone can view any transaction. Addresses aren’t labeled with names, which makes bitcoin pseudonymous rather than anonymous.

Here’s a simple example. Say Alice wants to send 0.01 bitcoin to Bob. Her wallet creates a transaction, signs it with her private key and broadcasts it to the network. Nodes check that the signature is valid and that the coins haven’t already been spent, then hold it in a waiting area until a miner includes it in a block. Once it’s in a block, the payment has one confirmation. Each later block piled on top adds another, making the payment progressively harder to undo, which is why merchants and exchanges often wait for several confirmations before treating a large payment as final.

Proof of work and mining

If no one is in charge, who decides which transactions get added? Bitcoin’s answer is proof of work. Miners, computers running specialized hardware, bundle pending transactions into a candidate block and then race to find a number that, combined with the block’s data, produces a hash below a target value. The only way to find it is brute-force guessing, which takes enormous computing effort. Checking the answer, by contrast, is instant.

The first miner to find a valid solution broadcasts the block, other nodes verify it, and the miner earns a reward: newly created bitcoin plus the fees attached to the transactions in the block. This process does three jobs at once:

1. It orders transactions, so everyone agrees on a single history.
2. It issues new coins in a predictable way.
3. It secures the ledger. Rewriting past blocks would require redoing their work and outpacing the rest of the network, which becomes prohibitively expensive.

The network adjusts the difficulty of the puzzle every 2,016 blocks, about every two weeks, so that blocks keep arriving roughly every 10 minutes no matter how much mining power joins or leaves. The trade-off is energy. Proof of work consumes large amounts of electricity, which is a persistent source of criticism.

The 21 million cap and the halving

Bitcoin’s monetary policy is written into its code. When the network started, each new block paid miners 50 bitcoin. Every 210,000 blocks, roughly every four years, that reward is cut in half, an event called the halving.

Approximate halving date	Block reward afterward
Launch, January 2009	50 BTC
November 2012	25 BTC
July 2016	12.5 BTC
May 2020	6.25 BTC
April 2024	3.125 BTC

Because the reward keeps halving, total supply approaches but never exceeds 21 million coins, with the last fractions expected to be mined around 2140. Each bitcoin is divisible into 100 million units called satoshis, so the cap doesn’t limit how small a payment can be. Supporters see this fixed supply as a hedge against inflation. Critics note that scarcity alone doesn’t create stable value.

Wallets and keys: what you actually own

You don’t hold bitcoin the way you hold a dollar bill. What you hold is a private key, a long secret number that proves you control coins recorded at certain addresses on the blockchain. A matching public key and address let others send coins to you. As Bitcoin.org explains, the signature made with your private key is what authorizes a transaction.

A wallet is simply software or hardware that manages those keys. The main options:

• Custodial wallets, typically on an exchange, hold the keys for you. They’re convenient, but you’re trusting the company, and exchange failures and hacks have cost customers dearly.
• Software wallets on your phone or computer let you hold your own keys.
• Hardware wallets keep keys on a dedicated offline device, which reduces exposure to malware.

Self-custody wallets usually give you a recovery phrase, a list of 12 or 24 words that can restore your keys. Anyone who gets that phrase can take your coins, and if you lose it along with your device, no help desk can restore access. That’s why seed phrases are a prime target in crypto scams.

Volatility and risks

Bitcoin has gone through repeated cycles of dramatic gains followed by steep crashes, with drawdowns of well over 50% more than once. Our explainer on bull vs bear markets covers how such cycles work in general. The SEC’s crypto assets resources highlight several risks investors should weigh:

• Price volatility, driven by sentiment, liquidity and news rather than cash flows.
• No deposit insurance. Bitcoin held at an exchange isn’t protected the way bank deposits are.
• Irreversible transactions. A payment sent to the wrong address or to a scammer generally can’t be reversed.
• Platform risk, including hacks, fraud and bankruptcies of crypto firms.
• Fraud and manipulation in less regulated corners of the market.

Volatility is also why bitcoin works poorly as everyday money for most people, and it’s one reason stablecoins emerged. Our explainer on stablecoins covers how they try to hold a steady value.

How bitcoin is regulated

Regulation varies by country, and it continues to evolve, so treat this as a general outline. In the US, no single agency oversees all of bitcoin. Commodity, securities, banking, anti-money-laundering and tax rules can all apply depending on the activity. Exchanges that serve US customers are generally required to verify identities and report suspicious activity.

A milestone came on January 10, 2024, when the SEC approved the listing and trading of several spot bitcoin exchange-traded products, funds that hold bitcoin directly and trade on stock exchanges. That gave investors a way to get bitcoin price exposure through a regular brokerage account, though the price risk is the same. The SEC’s chair stressed at the time that the approval wasn’t an endorsement of bitcoin itself.

For taxes, the IRS treats digital assets as property, so selling, trading or spending bitcoin can produce a taxable gain or loss.

The bottom line

Bitcoin is a decentralized digital currency secured by proof of work, recorded on a public blockchain and limited to 21 million coins by code. Its design removes the need for a central authority, but it shifts responsibility to the user: whoever controls the private key controls the coins. It has also been one of the most volatile major assets ever traded. Understanding the mechanics, the custody choices and the risks is the essential first step before deciding whether it belongs anywhere near your money.

For more explainers on digital assets and payments, visit our Crypto & Fintech hub.