Workflow Rules are older automation tools that can only handle simple if/then logic with limited actions like field updates, email alerts, and outbound messages. Process Builder is more advanced since it supports multiple criteria nodes, can create records, update related records, call Apex, and handle more complex business logic. However, Salesforce is retiring both of them in favor of Flow, which is now the recommended tool for all new automation. If I’m working in a legacy org where Workflow or Process Builder already exists, I maintain them but do not build new ones. For new automation, I always use Flow because it gives full control, screen flows, record-triggered automation, loops, decision logic, and integrations.