They have an active and passive status.
Passive means they can't be attacked and gives you a marginal bonus (bonus to skill, extra language, etc).
Active means they can affect combat in some meaningful way (i.e. attacks can originate in their square, foes don't have concealment when adjacent to it, etc.)
Each animal gives a different active/passive bonus.
There are also a slew of feats that you can take that grants additional abilities that are sort of like multi-class swap feats. There's one for example that can replace your utility feat (I think 6th level). It'll allow your familiar to sacrifice itself to take your damage for you for one attack.
Your familiars are more spirits/energy than real flesh and blood animals, so they just reform after a rest.
Most of them are pretty flavorful, and come with fun role playing quirks.
They also run the gamut from normal animals (bats, cats, ravens) to the unusual (flaming skull, fire demon, etc).
Cedric