I haven't got all of the demons that have ever featured in Buffy in here for 2 reasons, it would take me forever to write them all and you'd probably get bored of reading them.
The Master
Sire of Darla. Leader of the Order of Aurelius. He was so powerful that when he died his bones stayed intact. Buffy later crushed them with a sledgehammer.
Darla
Sire of Angel. Member of the Order of Aurelius. Darla was staked by Angel in order to save Buffy. In the 1st season finale of 'Angel' she is resurrected in human form. Drusilla turns her.
Moloch
After being bound in a book, Moloch had no form. When Moloch awoke in modern times he gathered acolytes around him, including Sunnydale students Fritz and Dave. He also used the internet to begin a relationship with Willow.
The Brotherhood of Seven
The Seven took the form of humans, but could only maintain these forms if they ate a human heart and brain every 7 years.
Drusilla
Sired by Angel, she sired Spike. Formerly girlfriend and lover of Spike she left him for a chaos demon and again later for a fungus demon. She has both visions and the power of hypnosis. She's slightly insane and has been known to have hallucinations.
Machida
Machida was worshipped by the fraternity brothers of Crestwood College's Delta Zeta Kappa for decades.
Eyghon
Eyghon was called up by Rupert Giles, Ethan Rayne and some of their friends. One of those friends sacrificed their life to return Eyghon to his dimension. But they all had the mark of Eyghon on them so when Eyghon returned to this plane he went after them all.
Der Kindestod
Der Kindestod was only visible to children, madmen and the feverishly ill. It spent its life feeding on children. It was also hinted that it had been the real reason for Buffy's cousin, Cecila's death.
Acathla
Acathla was capable of creating a vortex which would swallow the world. Centuries ago he was beaten by a knight who stabbed him. The demon turned to stone.
The First
The First is exactly what is says, the first, evil on earth that is. The precursor to all subsequent forces of darkness in the world.
Mr Trick
He was employed by Kakistos then by the Mayor. He was then killed by Faith who took his place. He was responsible for 'Slayerfest '98.'
Telepaths
These demons have no mouths and communicate with one another telepathically. When Buffy killed one of the demons and it bled on her, she was infected with its telepathic power until the blood of the second was used to cure her.
The Mayor
The Mayor employed and worked with many demons but the most memorable relationship was with Faith. He became a father figure, he cared for her deeply which was his undoing. His goal was to bring darkness to the world by reaching pure demon form.
Sunday
Sunday is the first vampire buffy meets after starting college. She isn't particularly strong for a vampire but manages to beat the Slayer ina fight, however the next day the Slayer dusts her.
The Gentlemen
The Gentlemen are from a fairytale. They come to Sunnydale and steal all the voice,s they are then able to harvest 7 human hearts, the Slayer stops them by breaking the box that holds the voices and screaming. Their heads explode.
The Initiative and Adam
The Initiative are a military organisation that capture demons and do experiments on them. The most prominent case of this is the chip they implant in Spike's head, this chip prevents a vampire from harming any human. Maggie Walsh created Adam, part demon, parrt human and part machine. Adam tries to create a half human, half demon army to take over the world.
Dracula
Dracula is introduced at the beginning of Season 5 and is pretty much as one would expect expect for the fact he is a lot easier to kill. He is successful to a point of getting Buffy under his thrall but our favourite Slayer isn't as easy to control as he obviosuly thinks.
Glory
Glory is an actual hellgod trying to get back to her own dimension. She is very strong, her only problem is that when she was banished from her dimension she was forced to share a vessel, a human male's body. Ben the orderly. To kill her Ben must be killed.
The Trio
The Trio is three boys playing with magic in an attempt to take over Sunnydale. Their magic proves to be more of an annoyance than a large threat until one of them turns to more human methods of attack.