Backend & systems·Transactions and isolation
you pick how much of other people's in-flight work your transaction is allowed to see, usually by accepting whatever the default was.
Isolation level
The setting that determines which concurrency anomalies your transactions can observe, from read uncommitted through to serialisable. Weaker levels are faster and allow more anomalies; stronger levels serialise more and produce more conflicts or blocking. Nearly every database ships a default weaker than serialisable, so the anomalies it still permits are yours to handle in application code.