The Orbeon Forms XForms engine proposes out of the box a set of controls, including input fields, radio buttons, etc. Those are typically implemented natively within the XForms engine.
Beyond the basic set of controls, there is an obvious need for creating new reusable controls. It would be difficult to modify the XForms engine itself each time a new control is needed. Orbeon Forms therefore supports a complete framework inspired by the XBL 2 specification to address this need.
NOTE: The XBL 2 specification is no longer under development at W3C, but as of 2015 Web Components embody most of the ideas of XBL 2, including custom elements, the shadow DOM, and strong encapsulation.
You can use components to implement:
Controls for datatypes which have a native implementation, but where a custom appearance is required
Example: A custom control for entering a date with dropdown menus rather than a date picker
Controls for datatypes which do not have a native implementation
Example: A control to capture the
Controls which do not have a standard XML type
Example: A phone number control
Controls which wrap existing controls
Example: A character counter component
Example: A map component
This is not an exhaustive list. Your imagination is the limit!
You might want to also check the components provided out of the box by Orbeon Forms.
Component or custom control: a piece of software which provides reusable behavior and presentation.
Component instance: a particular use of a component within an XForms document. A component might have multiple instances in a given page.
NOTE: This should not be confused with XForms instances.
Component implementation: the code which constitutes the inner workings of a particular component.
Component author: the person who writes a component.
Component user: the person who uses a component.
In general, writing a component will be harder than using one.
Obviously the user can be the same as the author!