Yesterday I recieved Spirit Conjuring for Witches: Magical Evocation Simplified by Frater Barrabbas and The Witches' Book of the Dead by Christian Day. I haven't read it yet, still waiting on two more books to arrive. These books discuss about spirit invocation and evocation.
Spirit-keeping is a termed coined by creepy hollows usually involves a vessel to trap a spirit to do your bidding, often the term is switched to companion if the spirit voluntarily bonded with the vessel, this vessel is used as a direct link to communicate with the spirit.
A servitor is a creation of Chaos magick, a spirit like being formed of the energy and intention of its creator that does not have a Will of its own but is bound to the will of its creator. They are advanced thought forms.
A vessel is made to contain something usually made of any hollow objects such as jars, bottles, gemstones, crystals, jewelry, dolls, etc.
A human spirit can also be used as a vessel.
Here's a thread about gatekeepers