And also whether they each do something that’s unique only to them. (Off-topic, the shade has to be my favorite familiar compared to the rest, with the spider being my second favorite.)
Here are the places you can get them:
You can get familiars at the start of LH2 (different ones for the 5 possible starting areas) if you choose to be a level 2 Druid, Ranger, or Wizard.
You get the Shade familiar if you study the Book of the Dead and become a Necromancer in LH2.
In LH3 you can make the Monkey your familiar if you have a class level in Wizard, Druid, or Ranger (you do not have to be Level 2 of those classes to get a familiar in LH3) and lack a familiar already.
If you get the Demon Master class (and have no familiars, I think) in LH3 you get an Imp familiar.
Different familiars have different combat capabilities. My personal favorite is the Firedrake. It is a good all-rounder and goes particularly well with the Dragon Rider class.
Sadly, you can only get one Familiar in the LH Trilogy, however there is a case where the MC obtained 3-4 Familiars (If Dragon counts as one) in The Last Wizard.
Yes yes it does