Here's a quick rundown on the cast (at least the major players):
Jensen Ackles (Dean Winchester) -March 1, 1978- Dallas, TX - Was on a daytime soap,Days of our Lives, before going primetime as a regular on FOX's Dark Angel in the 2nd (and last) season. Was a regular in the fourth season of Smallville, playing Jason Teague. Notable guest appearances include Dawson's Creek and Cybil. Movie credits include direct to video Devour, independent film Ten Inch Hero, and the horror remake of My Bloody Valentine.-Filmography at IMDb.com
Character: Dean is the oldest brother, and followed behind his father, John, into hunting demons, while his brother, Sam, tried getting away from it all. Dean takes it upon himself to look out after his brother, even sacrificing himself in order to bring Sam back from death at the end of the second season. The third season was all about Dean having a year, before going to Hell. The fourth season saw him being pulled out of Hell by the Angel, Castiel, in order to help prevent the rise of Lucifer.
Dean, himself, loves his car, rarely letting his brother drive it, he is a classic rock fan, doesn't understand iPods (he still uses cassettes in the car), and hooks up with a girl whenever opportunity presents itself (even a former Angel).
Jared Padalecki (Sam Winchester) -July 19, 1982- San Antonio, TX - Won the Claim to Fame Contest in '99 and got to appear on the Teen Choice Awards. In 2000, he got the role of Dean on the WB series Gilmore Girls, for which he was a regular from 2000-2004. Notable guest appearances includes ER. He appeared in several movies, including New York Minute, Flight of the Phoenix, House of Wax, Cry Wolf, and Friday the 13th. -Filmography at IMDb.com
Character: Sam is the youngest, and tried getting away from the "Family Business" by going to Stanford to be a lawyer. But, after his girlfriend (and was to be fiance) was killed in the same manner as his and Dean's mother nearly 22 years before, he went with Dean to hunt the things that go bump in the night. Sam was fed demon blood as a baby, so possesses supernatural abilities of his own (first being able to see visions related to the Yellow Eyed Demon, then a brief moment of telekinesis), later, taught to him by a demon, Ruby, he was able to exorcise demons, increasing survival of the host. He was more reserved but has started changing his personality, even behaving a little like his brother, almost taking a "love em, leave em" approach when it comes to women. He's hooked up with a werewolf and a demon (Ruby). The Angels dislike him, seeing him as a threat, as a demon gave him those abilities for purpose to run an army.
Jeffrey Dean Morgan (John Winchester) -April 22, 1966- Seattle, Washington- Most known in TV world as being a favorite dead guy. First, portraying Judah in Weeds, then dying after sacrificing himself for Dean's life on Supernatural, and winning hearts of females nationwide as Denny on Grey's Anatomy who still appears after death in Izzie's hallucinations. In the movie world, he's appeared in PS I Love You, and is in the highly anticipated graphic novel to movie adaptation, Watchmen. -Filmography at IMDb.com
Character: John, (Sam and Dean's father), was a Marine, and just a regular father, until November of 1983, his wife was pinned to the ceiling, and burst into flames. Since then, he went on a rampage to hunt demons and find the one that took his wife's life away, taking Sam and Dean with him wherever he went. He went on his own leads, leaving Dean to keep him out of the way, but that didn't stop Dean from getting his Sam from school, and them doing their own hunting while trying to find their father. They eventually met up, and separated only to rejoin with a special gun to kill Yellow-Eye. Unfortunately, a demon possessed a truck driver, crashing the Impala they all were in, putting Dean into limbo, until John made a deal to save Dean's life, at the cost of his own.
Jim Beaver (Bobby Singer) -August 12, 1950- Laramie, Wyoming- Beaver was on the critically acclaimed Deadwood, as well as appearing on many other shows over the years from John from Cincinnati, CSI, and Big Love, to Thunder Alley, Home Improvement, and 3rd Rock from the Sun. -Filmography at IMDb.com
Character: After Sam and Dean's father's death, Bobby became a father figure of sort for them, and helped them through all the rough patches. He was a hunter before John, and taught him a few things as well. He knows his way around the supernatural, even made his own demon-proof panic room. It was revealed in the third season, he got into the "hunt" because his wife was possessed by a demon, leaving him no choice but to kill her, as he wasn't aware of exorcism spells yet. His junkyard has proven useful, especially after the crash, it allowed Dean to work on his baby (the Impala), and now it's working as if nothing bad ever happened.
Genevieve Cortese (Pictures Left) (Ruby #2) -January 8, 1981- San Francisco, CA- Most known as playing Kris Furillo on the ABC Family show Wildfire for its four season run. -Filmography at IMDb.com
Katie Cassidy (Pictures Right) (Ruby #1) -November 25, 1986- Los Angeles, CA- Daughter of David Cassidy, she's appeared in several movies, Black Christmas, When a Stranger Calls, and Click. -Filmography at IMDb.com
Character: Ruby was a witch in her human days, and gave her soul to a demon. She escaped from Hell when the Gate was opened at the end of the second season, and is different as she helps the Winchester brothers, while other demons would want their heads on a stick (or something like that). It is revealed all demons were once human, and Hell changes them, ripping their souls, but she till remembers what it was like being human. At the end of the third season, she was sent back to Hell, only to be given another chance by Lilith, only to go against Lilith again and help Sam with his abilities. In order to gain Sam's trust, she found a coma patient and waited for the body to be "emptied" and took it, leaving the other person behind. Dean isn't too fond of her, but she proves useful every now and then.
Misha Collins (Castiel) -1974- Massachusetts - Best known as season one bad guy, Alexis Drazen, on 24. Also appeared on Charmed, CSI, and ER. -Filmography at IMDb.com
Character: Castiel is an Angel, the one that pulled Dean out of Hell in order to help stop the apocalypse. He tried communicating with Dean, even caused a psychic to go blind when trying to find out what it was that save Dean, and eventually inhabited a willing body (a believer willing to serve). Though he'll never really say it, but several times his face has hinted he's doubting his position.