servitor is essentially a thought form created, empowered and programed to a specific purpose. Provided that you have created it properly and programed it to the correct purpose it can aid you in your ritual or magical work, protect you or your home/magical space/temple, or a host of any other things. Servitors are not independent entities like elementals or spirits, although they can be created to have an independent existence, replenishing itself and in essence becoming self perpetuating. They are not naturally occurring entities but created beings for specific purposes.
There are several resources that are out there to teach you how to create servitors. Some of them are good, and some are downright dangerous, magically speaking. One of the best is dome by Andrew Martini on his Seventh Phoenix channel on Youtube, I highly recommend his method if you are looking at working with a servitor.
I do not typically invoke or evoke servitors, as mentioned by Shawn they are not like typical entities and are much more connected to your energy/ aura. Servitors do their function all on their own, we just supply the energy (usually). essentially it is doing it's job just by existing if it was created properly.