What might be in a directory?

• A geographical description of a county
  Including its geology, soils, agriculture, industries, transport, etc.
• The history of the county
• Descriptions of the individual towns and villages
  Including the amenities such as schools, churches, hospitals, work houses, etc.
• Lists of the inhabitants that have trades, and in most of the later directories even street by street, house by house lists of all householders.

A set of the 1791 Universal Directory

Is my ancestor going to be found in a directory?

Not necessarily. The early directories included people with trades. This doesn't mean just businesses and shops, but anyone with a recognised trade, such as a farmer, a sawyer, a teacher, or a dress maker working from home. Apprentices  and laborers were seldom included, although in the later post 1900 directories, that had street listings of people in major towns, such people were included. In any case, only the head of household was listed in a directory, not wives or children, and not lodgers.