You gain a familiar.
Prerequisites: Knowledge (arcana) 4 ranks, arcane caster level 3rd.
Benefit: You can obtain a familiar in the same manner as a sorcerer or wizard. As with a sorcerer or wizard, obtaining a familiar takes 24 hours and uses up magic materials worth 100 gp.
For the purpose of determining familiar abilities that depend on your arcane caster class level, your levels in all classes that allow you to cast arcane spells stack.
Source: Complete Arcane, page 81.