All-or-nothing change set transaction
Key Takeaway
A change set deploys all or nothing: one failing component means none of the components are committed. After a deployment succeeds, you can't roll it back.
When you deploy a change set, the whole change set runs as one transaction. Either every component deploys and is committed, or, if anything stops the deployment, nothing is committed and the org stays as it was.
A change set deploys in a single transaction
You deploy an inbound change set as a whole, not component by component, so you can't choose to deploy only some of its components.
The change set deploys in a single transaction, so the outcome depends on whether the deployment can complete.
If the deployment can't complete for any reason...
The entire transaction is rolled back, so one failing component means none of the components are committed. A failed change set never leaves the target org half-updated.
If the deployment completes successfully...
All changes are committed to the org and are permanent. You can't roll back the deployment, because the rollback protection only covers the deployment while it runs.
Think of a change set deployment like a bank transfer: the money moves completely or not at all, never halfway. The money stands for the components, so either all of them are committed to the target org or none of them are.