What Is Cryptocurrency? Blockchain, Nodes and Consensus
Lesson 1 of 10 · Beginner
What is cryptocurrency? A shared ledger
You do not need to be a programmer to understand cryptocurrency. Picture a ledger that records who owns how much money and who paid whom. At a bank, that ledger lives on the bank's own servers, and only the bank is allowed to write in it. If the bank makes a mistake, closes your account or goes offline, you have no recourse except to go back to that same bank.
A blockchain is that same ledger with one important difference: instead of a single copy in a single building, thousands of identical copies are kept on independent computers around the world. Anyone can hold a copy and read every transaction. A cryptocurrency (or crypto asset) is the unit of account that moves around in this ledger, such as bitcoin on the Bitcoin network or ether on Ethereum.
The key point is that the ledger does not record an “account” in your name. What exists is an address, and whoever holds the private key for that address can spend from it. That is why, in crypto, ownership means control of the key, not holding a certificate or an ID card.
Blocks, chains and hashes: why the past cannot be quietly rewritten
Transactions are not added to the ledger one at a time. They are collected into bundles called blocks. Each block has three main parts:
- a list of new transactions,
- a timestamp and some technical data,
- the fingerprint of the previous block, known as a hash.
A hash is the output of a mathematical function that turns any data into a string of fixed length. If even one character of the input changes, the hash changes completely. Because every block contains the hash of the block before it, the blocks lock together like links in a chain, which is where the word “blockchain” comes from.
In practice: if someone tries to alter an old transaction, that block's hash changes, the link to the next block breaks, and every other copy of the ledger immediately sees the mismatch. To make the change stick, the attacker would have to rebuild that block and every block after it, and also persuade a majority of the network to follow, which is extremely expensive on large networks. This is why a transaction buried deeper in the chain, with more blocks built on top of it, is harder to reverse.
Nodes and consensus: who writes the next block?
The computers that keep a copy of the ledger and enforce the network's rules are called nodes. Every node checks every transaction and every new block for itself. Is the signature valid? Did the sender actually have this amount? Has the same money been spent twice? A block that breaks the rules is rejected, no matter who sent it.
But with thousands of independent nodes, one question remains: who gets to add the next block? The answer is the consensus mechanism. Two common designs are:
- Proof of Work: miners compete by spending electricity and hardware to solve a computational puzzle, and the winner proposes the block. Bitcoin uses this approach.
- Proof of Stake: validators lock up the network's own asset as collateral and lose part of it if they cheat. Ethereum has used this approach since 2022.
Both aim at the same thing: making cheating costly and coordinating thousands of nodes that do not trust each other, without needing a central administrator.
Coin, token and key: three concepts not to mix up
A coin is the native asset of a blockchain, such as bitcoin on the Bitcoin network or ether on Ethereum. Network fees are usually paid in that coin. A token is an asset created by a smart contract on top of another blockchain. For example, Tether (USDT) exists as a token on several different networks. The difference matters because a token follows the rules of its creator's contract, and that contract can include features such as freezing addresses.
A private key is a very large random number. A public key and then an address are derived from it, and going the other way is practically impossible. When you send a transaction, your wallet signs it with the private key, and nodes verify the signature without ever seeing the key itself. If you lose the key, no support team can recover it for you. If someone else sees it, they can do everything you can.
What “decentralised” does and does not guarantee
The word decentralised is heavily marketed, so it helps to be precise about what it means.
What a large, genuinely decentralised blockchain usually provides:
- a ledger that no single entity can alter on its own,
- transparency: transactions can be inspected by anyone,
- supply and fee rules that do not change without broad consensus.
What it does not guarantee:
- Price: sharp volatility is part of the nature of this market, and no technology guarantees value.
- The safety of your wallet: if your key or recovery phrase leaks, the network just sees a valid signature and executes the transaction. There is no “undo” button.
- Neutral tokens: a token such as USDT has a centralised issuer that can freeze addresses, even if the underlying network is decentralised.
- Unrestricted access: exchanges, apps and infrastructure services are centralised. For example, Decrypt reported in March 2022 that MetaMask's default infrastructure provider blocked transactions from users in sanctioned regions.
- Legality or tax treatment: these are set by the law of each country, not by network code.
So decentralised means the ledger has no single owner. It does not mean your assets are risk-free.
Common mistakes
- Equating “decentralised blockchain” with “safe money”; the security of the ledger has nothing to do with the security of your key or the stability of the price.
- Assuming the asset is registered in your name and can be recovered with an ID document; on a blockchain only the private key matters.
- Mixing up coins and tokens; tokens such as USDT have a centralised issuer and may be freezable.
- Believing a wrong transaction can be “cancelled”; a confirmed blockchain transaction is not reversible.
- Trusting the trading volume shown on any website as a sign that a coin is credible.
What do volume and data say here?
The biggest difference between crypto and traditional markets is that the main ledger is public. Anyone can use an explorer (a blockchain browser) to see how much an address holds, what transactions it has made and when each block was recorded. That means you never have to take anyone's word about a deposit or a balance; you can check it yourself.
But keep two kinds of data apart:
- On-chain data: transactions that were actually recorded on the blockchain. They can be verified and cannot be changed.
- Exchange trading volume: buying and selling inside centralised exchanges often is not recorded on the blockchain at all and exists only in the exchange's internal books. The exchange itself reports that number. An NBER study estimated, in a 2019 sample, that on average more than 70 percent of the volume reported by unregulated exchanges was wash trading (fake trades).
The practical takeaway: for “what actually moved”, look at the explorer, and for trading volume, rely on a few large, reputable exchanges rather than the flashy numbers of unknown platforms.
📊 See real gold futures volume behind these ideas: free order flow course · today's gold volume map
Frequently asked questions
What is cryptocurrency in plain language?
It is a unit of account that moves around in a shared, public ledger called a blockchain. Thousands of independent computers keep that ledger, and ownership is determined by holding the private key, not by a name or an ID.
Can a blockchain be hacked?
The ledgers of large blockchains are extremely hard to tamper with, but most thefts happen elsewhere: leaked keys, fake apps, exchanges and cross-chain bridges. A secure ledger does not mean a secure wallet.
Is buying and selling cryptocurrency legal?
It depends on the country. Rules on trading, taxes and reporting vary widely, and some jurisdictions restrict or ban it. Check the law where you live; this course teaches mechanics and risk only.
Education only, not a recommendation to buy or sell; we never give signals. Price examples are illustrative. Test any method on a demo account first, with strict risk management.