Blockchain transaction

Every transaction contains the same building blocks, arranged differently by each chain. On Bitcoin it lists inputs, previous outputs being spent, and new outputs with amounts and destination scripts, and the fee is whatever is left over. On Ethereum it names a sender, recipient, value, data for contract calls, gas settings and a nonce. In both cases the owner signs it with a private key, proving authorisation without revealing the key.

Its life follows a common path. The wallet signs and broadcasts it, nodes validate it and keep it in their mempools, a miner or validator includes it in a block, and further blocks add confirmations until it is considered final. The transaction hash, or TXID, lets anyone track each step on a block explorer.

A transaction is not reversible once confirmed, and no help desk can cancel it. Before confirmation it can still be stuck, replaced or dropped. A transaction can also be included but fail, as when a smart contract call reverts, and still charge the gas used. Address errors, wrong networks and malicious signatures are the most common ways users lose funds.