Azazel is the main antagonist of season 1 and season 2.
In the Judeo-Christian Bible, the name "Azazel" is Hebrew in origin and means "for the complete removal" and is referred to in the scapegoat rite. In some traditions of Judaism, Christianity, and Islam, Azazel is the name for a chief fallen angel. According to a book of the Apocrypha, the Book of Enoch, Azazel taught men to make swords, knives, shields, and breastplates as well as metalworking, jewelry making, and other decorations and body adornments. He was then cast out of heaven by the four archangels.
It is quite possible since Azazel is a fallen angel in lore, and he is a fallen angel in the Anime Series, that Azazel was an angel that fell with Lucifer, lost his grace, and was twisted into a demon. This is further supported by the fact that even prior to his arrival, cattle deaths, electrical storms and temperature fluctuations occurred, and his presence has caused electrical devices to go haywire. He has also displayed the ability to possess reapers, a type of angel. No other demon has caused these effects by mere presence or displayed an ability to possess reapers. It is unlikely that he was twisted into a demon by Lucifer himself, as he is distasteful towards demons, and thinks of angels as the highest species.
In lore, Azazel is known for creating acts of chaos and mischief. This information was retrieved from the Extended-Scenes on Season 4 DVD of Supernatural.
In a discarded version of a script, Dean once referred to him as "the Big Bad", which is a popular term to denote the main antagonist in each season of Buffy The Vampire Slayer.
Azazel made #6 of the Top 10 Scariest Demons from TV (By Dean Bextor, Friday, December 12, 2014) It says: The Yellow-Eyed Demon may not be the most powerful demon on Supernatural but he is terrifying nonetheless. He's the first demon featured on the series and he made the biggest impact on the Winchester family. He created special children who would take part in the war against humanity and more importantly, he's responsible for the death of Mary Winchester which basically served as a catalyst for the entire show.
He was one of the few characters to actually teleport on-screen, when Sam tried to shoot him with the Colt in Salvation.
He remains the only known yellow-eyed demon until season 12. However, it is interesting to note that after season 2, his eyes change in their actual appearance.
He was the first of four main antagonists to be killed by Dean. The second one was Eve, the third was Dick Roman, and the fourth was Abaddon.
He is the first of three characters to be the main antagonist for two seasons. The second and third are Lilith and Crowley, in order.
Azazel is the first demon of lore to appear in the show.
As Azazel had met and been one of Lucifer's closest demons before he was imprisoned. Azazel is one of the oldest demons to appear on the show, along with Lilith, Abaddon, Cain, and Samhain.
He is the first supernatural creature to appear in the show.
"The yellow-eyed demon and Meg are so much fun to write. The yellow-eyed demon really made his first appearance in 'Devil's Trap'. He was around, but he was never a character who had dialogue and personality. There's a movie called Devil's Advocate, and the sense of humor that Al Pacino brought to his demonic role was great. We really liked the idea that in an episode that's so heavy, let's give the demon a personality and a sense of humor, and a sense of fun - he enjoys what he's doing. Nothing gives me more pleasure than writing one of the demons, because they can be so cruel and so funny; there's nothing they can't say. It allows you to be so mean, but have so much fun with it. Every insult you've always wanted to say, the demon can just say it because that's who he is. Between him and Meg, those two were just really fun characters to write because of how they could talk to the boys and really twist the knife psychologically." - John ShibanSupernatural The Official Companion Season 1, p. 115
Season 8 seems to be the last season he is seen, or mentioned.
Azazel has been portrayed by numerous actors, but all have maintained his sadistic sense of humor and Jack Nicholson-like mannerisms. Critics and fans alike have met him with nearly universal praise.