The defining word is ongoing. A break-fix shop is paid when something goes wrong, which means its revenue rises when the client's environment gets worse. An MSP is paid the same whether the week is quiet or chaotic, so the incentive reverses: every problem prevented is margin kept. That single change is what makes the model work, and it is why an MSP invests in monitoring, patching and standards that a break-fix shop cannot justify.
In practice the responsibility usually covers endpoints and servers, network equipment, email and identity, backup, and a service desk for users. Where it stops is a matter of contract rather than definition, and the boundary is worth reading closely — it is where disputes happen.
What an MSP is not is a staffing arrangement. Hiring an MSP does not buy a set number of hours; it buys an outcome, delivered by whatever effort that takes. Clients who try to measure the relationship in hours consumed are measuring the wrong thing, and providers who let them are usually the ones being consumed.