Per the description, it's a familiar that works just like a witch's familiar and has the spirit animal features on top.
Per the description of a witch's familiar (in the
Witch class), a witch's familiar works just like a wizard's familiar, except it also stores spells.
So it's just lke a wizard's familiar (always there, having it die is a problem, your link to it improves as you gain levels...), except for storing spells (so if it's gone or dead, you can't memorize or learn new spells) and having spirit animal features (so it's a but more capable than an ordinary familiar).
It's all in the text, really.
That said, if you're interested in summoning familiars, you may be thinking of the
Occultist third party class, which lets you pick a set of daily powers each day by summoning spirits (a different kind of spirit from those described in the wizard archetype - too many meanings for too few words). Typically, oneof the powers in the set can be traded for a familiar of some sort. That lets you summon a new 'familiar' every 24 hours, but it's a different class and options.
You can find the class here:
http://www.d20pfsrd.com/classe...ance-house/occultist
You can find the rules fof summoning Occultist spirits here:
http://www.d20pfsrd.com/magic/...ules/binding-spirits
You can find the list of spirits that an occultist can summon here:
http://www.d20pfsrd.com/magic/...ding-spirits/spirits
Sadly, you cannot take the Spirit Whisperer archetype AND the Soul Weaver archetype, which would have let you be a Spirit Whisperer wizard who can also summon Occultist spirits. This is because they affect the same class features.