Transaction

A Bitcoin transaction is a data structure that moves BTC from one set of owners to another. It does this by consuming existing unspent outputs (UTXOs) as inputs and creating new outputs that lock the funds to new spending conditions.

The structure is atomic: a transaction either succeeds entirely or fails entirely. Every input must produce a valid signature (or script witness) proving the spender controls the UTXO being consumed. If even one input fails, the whole transaction is invalid. The sum of input values must be at least the sum of output values; the difference is the transaction fee paid to the miner.

A typical send works like this: your wallet picks one or more of your UTXOs whose value adds up to at least what you want to send, signs them as inputs, creates an output paying the recipient, and creates a change output paying any remainder back to yourself. The whole package gets broadcast to the peer-to-peer network, where it sits in mempools until a miner includes it in a block.

The transaction is identified by its txid - the double-SHA256 hash of its serialized form, which since SegWit leaves the witness data out. Once confirmed, the outputs it created become new UTXOs in the global set, available to be spent in future transactions, and the inputs it consumed are gone for good.

See the Mining rabbit hole section 6 for how miners decide which transactions to pack into a block, and UTXO for the "coin object" model that underlies Bitcoin's accounting.