What is the difference between Workflow Rules and Flow Builder?

Workflow Rules are the legacy automation tool limited to field updates, email alerts, tasks, and outbound messages. They only trigger on record save and have limited cross-object capabilities. Salesforce has announced their retirement. Flow Builder is the modern replacement offering far greater capability: Record-Triggered Flows, Screen Flows, Scheduled Flows, and Auto-launched Flows. Flows can create/update/delete records on any object, call Apex, send custom notifications, post to Chatter, navigate subflows, and handle complex branching. Admins should migrate all Workflow Rules to Flows. Flow also replaces Process Builder, which is also being retired.