From ErfWiki

An Overlord is a Ruler that is not a Royal.

Proposed Canon

Known Overlords include:

• Stanley the Tool of Gobwin Knob
• Charlie of Charlescomm
• Overlord Firebaugh of Goodminton (http://www.erfworld.com/2011/10/inner-peace-through-superior-firepower-episode-001/)
• Wanda Firebaugh, briefly Overlord of Goodminton.
• Judy Gale of Haffaton

Speculation

Sides run by Overlords rather than Royals can't pop Royal or Noble units.