Glossary
UTXO
Unspent transaction output: the accounting model used by Bitcoin, where each transaction consumes prior outputs whole and emits new outputs.
Bitcoin doesn't have account balances. It has UTXOs — unspent transaction outputs. Each transaction picks one or more unspent outputs as inputs (consuming them entirely) and creates new outputs that the recipients can later spend.
Your "balance" is the sum of every UTXO whose locking script you can satisfy. The model is parallelizable and stateless per transaction, which makes for clean validation, but it is awkward for general computation — which is why Ethereum chose an account model instead.