Classic workflows is a part of Microsoft Dataverse and it can be used to add no-code business logic and automation.
There are three types of processes in Microsoft Dataverse. Classic workflow is one and the other two are Dialogs and Actions. All these three are based on Windows Workflow Foundation (WWF).
It is recommended to use Power Automate cloud flows instead of classic workflows. Cloud flows run asynchronously though, classic workflows can be run synchronously. Always consider to use Power Automate cloud flows instead of classic workflows. Another alternative is to create plug-ins, it requires writing code.
Workflows can be extened using code if no-code does not support what you want to achieve. Then a Custom Workflow Activity (CWA) can be created.