Hogwarts ghosts

• Nearly Headless Nick
• Moaning Myrtle
• Professor Binns

Ghosts are an unusual type of magical creature. In some ways, they're not creatures at all, but rather characters just like Harry, Ron, Dumbledore, and all the other living people in the books. But they aren't quite the same, since they've all had the misfortune to die unexpectedly and are now semi-transparent, non-corporeal beings. 

Ghosts can pass through solid objects. They do not eat, but in many other ways they seem to enjoy a full life. They interact with "live 'uns," as Sir Patrick said, referring to Harry, Ron, and Hermione. There are differences, however. It seems that their sensibilities are somewhat altered since their deaths, since their idea of music--an orchestra of thirty musical saws--is bizarre and dreadful to the ears of the living. Also, they do seem to be in some way tied to a place, some (Myrtle, for example) more than others. They don't all get along exactly well, either. The Bloody Baron is avoided by most ghosts, and the Hogwarts ghosts don't go into the Shrieking Shack because they say a "rough crowd" lives there (although this is likely a story they've invented to help support the official lore of the place).

While ghosts as a rule don't affect the physical world--people walk right through them, for example--there are some exceptions. Myrtle can splash water out of the toilet she haunts when she wants to cause a ruckus to demonstrate how miserable she is.

The Ministry of Magic has some authority over ghostly behavior. When Myrtle was stalking Olive Hornby in retaliation for the way Olive tormented her in life, the Ministry was called in and Myrtle was forced to return to Hogwarts and haunt the place of her death, the first-floor girls' restroom (GF25).

Deathday Party (CS8)

• Sir Patrick Delany-Podmore - leader of the Headless Hunt, with a group of his huntsmen
• group of gloomy nuns
• ragged man wearing chains

This ghost in chains could be Jacob Marley, the ghost of Ebenezer Scrooge's dead partner, from Charles Dickens' A Christmas Carol. Thanks to Mike Ball for this suggestion.
• knight with an arrow sticking out of his forehead

It is possible that this is the ghost of could be Harold II (1022? - 1066). He was the last Saxon king of England. He was defeated at the Battle of Hastings in 1066 by William the Conqueror. Harold was killed when an arrow pierced his eye. Thanks to Mike Ball again for this excellent deduction.
• portly ghost who was almost eating
• the Wailing Widow from Kent

"My whole body sort of seized up, and then I was floating away...And then I came back again. I was determined to haunt Olive Hornby, you see." (CS16, italics added)