A cryptocurrency is an entry in a shared ledger. The ledger records balances and the transfers between them, and it is maintained not by a bank or a clearing house but by every computer running the network's software. Each of those computers holds a full copy, and the rules of the software decide which new entries all of them will accept.
That single design decision is the source of nearly everything else that is said about the subject. Because no institution holds the ledger, no institution can freeze a balance, reverse a transfer or decide who may open an account. Because no institution holds the ledger, there is also nobody to call when a transfer goes to the wrong address, no deposit insurance behind a balance, and no password reset. The same property is described as censorship resistance by people who want it and as an absence of consumer protection by people who do not. Both descriptions are of the same mechanism.
Ownership is proved by a private key — a very large number that only the owner holds. A transfer is a message signed with that key, which the network can verify against the corresponding public address without ever seeing the key itself. This is why the standard warning is stated so bluntly: whoever holds the key holds the coins. A key written down and photographed is a key that has been given away, and a transfer authorised by a stolen key is, to the network, indistinguishable from a legitimate one.
The word “cryptocurrency” now covers things with very little in common. Bitcoin is designed as a scarce bearer asset with a fixed issuance schedule. Ether is the fee token of a general-purpose computing platform. A stablecoin is a claim on a reserve held by a company, engineered to be worth one dollar and to be boring. A governance token is a vote. A meme coin is a collectible whose entire value rests on other people wanting it. Grouping them because they share a settlement layer is like grouping a mortgage, a lottery ticket and a season pass because all three are sold on paper.
The practical consequence is that questions have to be asked about a specific asset rather than about the category. Who issues it, and can they issue more? What does holding it entitle you to, if anything? Where does its price come from — a redemption promise, a cash flow, a fee it captures, or purely what the next buyer will pay? Those four questions separate assets far more usefully than the technology does.
Two things are worth stating plainly at the outset because they are where new readers most often go wrong. First, a token being listed on a large exchange is a commercial decision by that exchange, not a verdict on the asset. Second, a blockchain guarantees that a transfer happened; it guarantees nothing at all about whether the thing transferred is worth anything. The ledger is honest about its own contents and silent about everything else.