From Diablo Wiki
"Caster weapon" is an informal term for weapons that can only be used by mage characters. Known caster weapons in Diablo III include daggers, wands, and orbs, though others, such as short staves, may also deserve to be on the list.
Caster weapons do not spawn with melee weapon mods, such as +damage. They are intended to be used to boost spell casting, not as melee weapons, though they can be hit with, in a moment of desperation.
Known Caster Weapons
The following caster weapon types have been confirmed as returning in Diablo III. These weapons were seen in gameplay movies, during play demos at Blizzcon, or were mentioned by the Diablo 3 developers during various interviews. 
- Daggers -- A wand-like mage weapon, distinct from swords in Diablo 3.
- Orbs -- A Wizard-only item that's held in the off-hand. Not used for blocking; just bonus properties.
- Staves -- Short staves are like larger wands, and can be held one-handed by spell-casters. Longer combat staves are Monk weapons.
- Wands -- Wizards and Witch Doctors only.
All are subject to change, as the itemization process continues.