"It might seem ungainly to carry a small instrument in the off-hand, but great power can be contained in a perfect sphere, inhuman skull or tome of forbidden script scrawled in the tongues of kingdoms past."— Game Guide(src)
Unlike Sorceresses, Wizards treat Sources as a supporting item, used to store and amass their magical powers, therefore enhancing their spells. While Sorceresses were usually combining their Orbs with shields for greater protection, Wizards combine Sources with other one-handed weapons instead, most often with Wands, mocking the combat tactics of the Sorcerers and going for maximum destructive power at cost of defense.
Most Sources take form of Orbs (usually detached from their shafts and gently levitating above the Wizard's palm instead of being held), but many Wizards are known to carry enchanted spellbooks, puzzle boxes, cursed skulls (sometimes even those of dead sorcerers), and other relics.
Note that Enchantress Focuses are different from Sources.
In game, Sources always provide a damage boost (added to the damage of the main-hand weapon, the amount is fixed and therefore boosts DPS much more if combined with faster weapons). This damage does not count as an affix, making a total of 7 properties available on most orbs.
Sources can roll the following stats (in addition to all-armor and core stats): reduced cooldown for all skills, specific skill damage, sockets, maximum Arcane Power, Arcane Power regained on critical hits, Critical Hit chance, increased damage against Elites, various 'on hit' effects and Area Damage. They count as Armor for the purposes of Socket gem stats.