In short, they are your soul. The spelling is taken from the His Dark Materials books. The concept in the books was taken from Aristotle's concept of a daimon who was his spiritual guide.

A daemon in our world is incorporeal, more of a voice in your head. When people visualize them, they are usually animals. They can shapeshift to almost anything. Eventually they will settle as an animal that reflects your personality and character the best.

A unix program that runs in the background of the system. Often signified with the -d suffix on the end of a program name.

Variant of daimon.
Daimon: "An attendant spirit"

"Computer Science: A program or process that sits idly in the background until it is invoked to perform its task."

the medieval spelling of the word demon

1.A type of demon in strict reference to a Greek definition rather than the often associated Christian interpretation. "Daemon" is actually the mis-romanization of the Greek word "Daimon" for there is no Greek dipthong for "ae". It is often used incorrectly by people attempting to sound pretentious, arrogant and self important who really no nothing of the subject they speak about.

A person who tells bad sexual jokes and wanks in public toilets

Unlike Demons, which are extradimensional creatures, Daemons are what are best described as negative beings, or walking vortexes of dark matter. They are beings of pure evil that are from a prallel universe. As soon as a gap is breached between our universe and theirs, a massive extinction often occurs. Daemons are unstoppable, except by the Demons and Gods.