A misunderstood term. There are many different faiths under satanism the most common is LeVayan Satanism, founded by Anton LeVay. It is attached to the Church of Satan and the Satanic bible. It is a more atheist (as they prefer to call themselves) point of view. Practicing rituals and spell writing and incantation relates it to Wicca or Pagan groups. Though the group practices magic, majic, magick, majik, or how ever you would like to spell it they do not believe in a celestial being or "God" except for themselves creating the philosophy "You are your own God" a point of view meaning you choose your own destiny not a false being claimed to have created all the existence from nothingness. Another View of satanism believes Satan was the true creator and during an inquisition the catholic church stole satanism and turned it into the opposing religion. These satanists usually practice the Ars Goetia, or the art of summoning of the 72 demons. They practice summonings, rituals, and spells found in LeVayan Satanism. Another for of satanism believes the bible and everything it says but choose to fallow Satan and his army of hell. These satanists are/where common on the black metal scene especially in the 90's. Satanism can be related to Christianity and Judaism because of its extensive branches each believing very many differences.
Satanism is by far the most misunderstood religions.