Widgets are reusable elements that contain complex and/or commonly used elements and workflows. You can use these throughout your app.
UI Widgets UI widgets offer elements designed to be interacted with by your users on the front end and contain workflows specific to the reusable. Typically you should clone these reusables and customize them for your specific use case.
Workflow Widgets Workflow widgets contain workflows that can be referenced from the page using the 'Trigger a custom workflow from a reusable element' action.