Non-Magic User, or NMU is a term used broadly to define characters who cannot use magic. Included are Thieves, Commoners, and Barbarians. These characters cannot learn spells, cannot use scrolls, wands, or runestones.
Barbarians are a special case. While they can use some of these items (notably, gwethdesuans and polo'izh hide cloaks), they are heavily limited and may be further restricted in the future.