Warning! This page contains a considerable number of spoilers.
Salem is really a warlock, who after attempting world domination, got sentenced to 100 years as a cat by the witches' counsel.
The reason Salem is a cat is because he's being punished. He was a witch like Aunt Hilda and Zelda. But he tried to take over the world and didn't succeed. His punishment was to spend 100 years as a cat with no magical powers. [more...]
We think he's been a cat for 25 years already, so about 75 are left. [more...]
Source: Nick Bakay (read the complete interview).
Salem was forced into confessing his Scheme for World Domination at the witches' counsel due to their use of truth sprinkles. Truth Sprinkles can be made from scratch (which involves preheating an oven to 5000 degrees) or can be bought pre-made (instants). (#002, "Bundt Friday")
The leaders of Salem's revolution were sentenced to be turned into
house pets. The peons who aided them were sentenced to take care of
Source: ANiMATO! (The Animation Fan's Magazine), Issue 39, Page 14
Monty is Hilda and Zelda's cousin. Monty's pet, Newt, was deep into Salem's world domination scheme: Salem had promised him Denmark. (#011, "A Girl and Her Cat")
During his 25th year as a cat (season one), Salem visits his witchly parole officer, and pleads that his rehabilitation is going well, but lets slip that he still dreams of world conquest. A flashback allows us to glimpse him in human form, and also reveals that Hilda was one of his followers: his punishment is to be a cat for a century, hers is to keep him worm-free for the same period. (#022, "The Great Mistake") As part of his punishment, Zilda has told Salem to do Community Service duties. He is a member of the "Pets for Prisoners" scheme. (#003, "Terrible Things")
In the Showtime Movie, Salem was turned into a cat because he tempted a mortal into having sex, and only got (around) 90 years. A bit weak, since Sabrina's mother is a mortal. This was changed into the above story for the series.
The cat was in the original Archie comic, Sabrina, but
was a Tabby cat. In the comic, Salem was turned
into a cat by a lovelorn sorceress he had rejected (who also turned
his girlfriend into a dog who was alergic to cats!). In the early 1970's The Archie Comedy Hour
and Sabrina and the Groovy Goolies, Salem was a
non-anthropomorphic cat with magical powers.
Sources: Melissa Joan Hart, ANiMATO! (The Animation Fan's Magazine), Issue 39, Page 14
Oddly, Salem wanted to attend Sabrina's parents evening. This was probably to later tease her, since he doesn't like to help even when he knows how - when Sabrina turns Rudy Kazootie into a fully grown man she asks Salem for help but he denies any knowedge of what she can do. He should know, however - he was once a sorcerer/witch himself. Even though he can't do any magic himself he can remember the spells, as demostrated earlier when he tells her about the sport cheat spells. Typically, he doesn't mind the use of these sport cheat spells - as long as he can benefit! (#004, "The True Adventures of Rudy Kazootie")
Salem is maxed out, cash-wise. His credit card was approved by the Good People of the Bank of Nurg. (#020, "Cat Showdown") It turns out that he has prayed for money in the past... in fact money seems to be a major motivation in his life (along with power of course). His money problem may have been offset slightly since he has invested the money he "stole" from the Aunts. (#026, "Sabrina Gets Her Licence")
Like a real cat, his tail puffs up when he is attacked or scared. (You can see this in #004, "The True Adventures of Rudy Kazootie", and briefly in #018, "First Kiss" when he stops Sabrina and Harvey from kissing)
Salem's memory must be quite good. In the first episode of the first season, Sabrina's 16th birthday (#001, "Pilot"), Salem asks Sabrina to "get this stupid hat" off his head. A year later, on her 17th birthday (#025, "Sabrina Gets Her Licence"), they wear party hats again - but this time they were his idea!
Before being turned into a cat, Salem told Patrick Henry that his (now famous) line needed more
punch - originally it was going to be "
Give me liberty or give
me some other option" (he changed it to "
liberty or give me death"). (#025, "Sabrina
Gets Her Licence")
Salem likes singing, and is quite good at it. In fact, in #034, "To Tell a Mortal", Salem none-too-modestly claims to have a great singing voice. In one episode, he sings "Witchcraft" to himself in the mirror (Sabrina and her aunts congratulate him). (There are versions of "Witchcraft" on Bill Evans' album "Portrait in Jazz" and Bobby Hutcherson's "In the Vanguard".) In #011, "A Girl and Her Cat", Salem also sings "God Rest Ye Merry Gentlemen".
Salem's advice to all young cat owners is "scratch behind the ears". (Source: Totally California TV programme) He loves his yarn, too ("I have urges, Sabrina!"). Another game he likes is Risk, which he plays with Sabrina. (#004, "The True Adventures of Rudy Kazootie") He doesn't like dogs though. (#025, "Sabrina Gets Her Licence")
Another of his hobbies is surfing the net - he is computer literate and types with a pencil. (#018, "First Kiss") When Salem can't sleep he reads boring books, like Sabrina's diary - which is locked, but that isn't a problem since he just opens it with his dew claw! (#002, "Bundt Friday") Note that he also used his dew claw to open Sabrina's magic deactivator when she escaped from witch camp. (#025, "Sabrina Gets Her Licence") Also, when he is thouroughly bored, he indulges in a bit of lint (fluff) spotting: "Pick out a piece of lint and track it throught the air". (#021, "As Westbridge Turns")
He likes Liver and Bree Omlet. (#025, "Sabrina Gets Her Licence")
If you are a fan of the comical side of Salem, then a few Must-See episodes are...
#011, "A Girl and Her Cat", where he is catnapped (note the brief appearance of Nick Bakay in this ep).
#017, "Sweet Charity", where Salem eats too much hair soup and becomes a long-haired cat.
#020, "Cat Showdown", where Salem enters a cat competition.
At the Greater Boston Cat Show (#020, "Cat Showdown"), Salem won Best Of Breed: American Shorthair and would have gone on to win Best Of Show if it hadn't been for Sabrina. Sabrina, however, named him Best Of Show privately the same evening.
In "Teen Beat's" 14th Annual Teen Star Awards , Salem was voted Fave Animal/Fantasy Star.
In the Nickelodeon's 11th Annual Kids' Choice Awards  Salem won "Favorite Animal Star". You can read the press release if you really want to.
This is, as far as I can tell, currently the only detailed web page concerning Salem. However, other pages do mention him.
Do you have more material? Pictures or sounds of Salem? Information? Corrections? Comments? Links? Please e-mail them to me: firstname.lastname@example.org.