It adds a +2 trait bonus to a single casting class that you choose, as long as it doesn't exceed your HD.
Does that bonus affect your familiar's statistics and abilities granted from your chosen school, or does it only affect spells per day (and spells known if you are a spontaneous caster)?
Actually, it does even less than your thinking. It is only a +2 trait bonus to caster level, meaning that it only impacts caster level dependent variables, and does not add to your spells known, casts per day, or anything else typically gained from a higher level in that class.