Warlocks bend demonic forces to their will, able to summon and command a variety of demons to serve their needs without falling fully into darkness themselves. This can also be seen in their ability to summon magical mounts to carry them.
Shaman are very spiritually oriented and can often see things and creatures in the spirit world unknown to most others. One prime example of familiar-type animals that assist the shaman is the spirit wolf; indeed, shaman can transform into them as well. Spirit wolves in most cases are used in battle to defend the shaman.
Shaman can also summon forth creatures from the Elemental Planes. These are usually fire or earth elementals. Unlike warlocks and mages that bend creatures' wills to do their bidding, shaman are in tune with nature and treat their summoned creatures as allies.
Mages with the frost specialization summon from the Elemental Planes to serve them in battle. All mages can summon a non-combat familiar to follow them, if they so choose to train .