A Faustian contract (i.e. deal with the devil) is a link and bargain between a human and a demon whereas the latter offers specific services and fulfills the individual human's wish in compensation of consuming the said human's soul.
For a demon: To consequently consume the human's soul.
For a human: Typically, to fulfill something tantalizing and difficult to accomplish, such as a satisfying revenge (e.g. Ciel wishes for those who betrayed the Phantomhive family to experience the same level of humiliation and suffering he did). Afterward, they are to give up their soul to the demon as payment.
Creating a ContractEdit
In order to create the contract, a Human has to summon a Demon. However, it seems that they do not need to be aware of the existence of demons, attempt to summon a specific demon, or even summon one intentionally, as Ciel Phantomhive states he summoned Sebastian Michaelis accidentally. It is implied that the human must be desperately searching for a way out, if Ciel's situation is indicative of a typical summoning process, and have little or no faith in God. Sebastian states that once summoned, this fact will not change for all eternity. All the same, it seems that the human still has the option to go through and create the contract, or dismiss the demon, as when Sebastian was summoned, he asks Ciel to make a decision. However, the creation of the contract seems to be fully completed as soon as the human issues a command to the demon.
William T. Spears compares a demon's contract with a human to poking fun at them and then leeching anything that comes out as a means of survival, to which Sebastian says he does not dismiss that claim.
Once the contract forms, the human bears a mark in ambiguous places (e.g. Ciel bears it on his right eye which he hides with his eye patch and Sebastian bears it in his left hand which he hides with his glove). In a sense, the contract is viewed as a 'scar' the demon leaves so the demon would not lose the person he contracted with. No matter where, the contract has the authoritative power; the demon is able to detect where the human is, and irrefutably, the latter can never escape the demon. Sebastian has correlated the contract to a 'collar'.
The demon seemingly is able to inflict the mark on the human by simply pressing their hand against the chosen place. The choice as to where the seal resides is up to the human as shown when Sebastian asks Ciel where he wants the seal to be located. It is noted that the more visible the place where the seal is, the stronger its power. The placing of the seal seems rather painful.
If the human's wish is not yet fulfilled, the usual practice for a demon is to serve under the human for as long as it takes and to perform supplementary orders; Sebastian and Claude served their rightful masters as butlers. They are to be the humans' power and to protect the humans for the sake of their demons' aesthetics.
Their principles basically states that they must follow and obey the orders their 'master' gave, (i.e. the human they contracted with), as long as the contract is still in effect.
Once the contract is completed and the human's wish is accomplished by his or her demon, the human's soul is eaten by the demon resulting in the human's death. Alternately, the human may be turned into a demon if that is what the contract states. 
|Human · Demon · Grim Reaper · Bizarre Doll · Ghost · Doll · Angel|