What's an XFO?
An XFO is an object, or group of objects, stored in the Object Library of Adobe LiveCycle Designer.
XFOs are the fastest, most efficient way to build forms in Designer. An XFO can be a single field, such as a Social Security or Tax ID number, a group of fields such as an address block, or an entire subform.
Where are My XFOs?
Your beginning set of XFOs are found in the Object Library of Designer. You can modify or add to the existing groups in your library, or create your own. In the example below, Carl Young has created a library group called "MY XFOs" in his My Documents folder. You can create shared groups on a network drive, too.

See Managing the Library palette in Designer Help for detailed steps on setting up Library groups.
XFO Examples
Samples available here include:
Instructions
Click on the link (a ZIP archive) and download to your computer. Extract the XFO object and place it in a directory accessible by Designer. Then add a new library group. Go into the properties of the group and point to the directory holding the XFO.