"Well, well, well, we are in trouble."
-- Argus Filch (PS14)
Argus Filch was the Hogwarts caretaker, a cantankerous, nasty man who detests the students. He knew the secret passageways and hidden doors of Hogwarts better than anyone except Fred and George Weasley (and now Harry, thanks to the Marauders’ Map).
Filch was a Squib, which means that he was born of a wizarding family but cannot do magic. The frustration this caused is partly to blame for his horrible and vindictive attitude. Hanging from the ceiling of his office were chains and manacles which he keeps polished up in case he was ever given permission to use them on students, something he would have dearly loved. Under Umbridge, Filch actually walked the halls of Hogwarts with a horsewhip (OP29). During the Battle of Hogwarts, Filch helped Madam Pomfrey evacuate the students from the school (DH31). There is no record that he stayed to fight.
Filch had a “scrawny, dust-colored” pet cat called Mrs. Norris with whom he had an almost psychic connection. She wandered the corridors of the castle just as he did, and if she spotted anyone putting a toe out of line, Filch was there within minutes.
Hogwarts: Hogwarts Caretaker; his office is probably on the ground floor.
Appearance: Rheumatic (OP28) and hunchbacked, Filch wheezes and shuffles when he walks (PS). He has pale eyes, jowls that quiver (PA8), and thin grey hair (OP14). He wears a brown coat (PA11).
Interests: Kwikspell courses (CS8), torturing students for infractions, giving detentions, and polishing the manacles hanging in his office.
Cleaning, enforcing rules, being Umbridge's lapdog.
Argus = giant in Greek mythology, a watchman with a hundred sets of eyes (200 in total).
Filch = English slang "to steal."
From the Web
Argus Filch from the Harry Potter Wiki
"What is the Point of Argus Filch?" essay by Mikaeka Renshaw on MuggleNet
"What does Argus Filch Specifically Do?" from SciFi StackExchange, provides a very nice concise list of the duties Filch has performed over the years