Pamela Sue Voorhees is a fictional character in the Friday the 13th films. She is the mother of Jason Voorhees, the main character of the series.
In the original Friday the 13th film, the character is simply named Mrs. Voorhees, her first name not being revealed until Friday the 13th: The Final Chapter.
The character was portrayed by Betsy Palmer in the first two films. During the character's brief appearance in Freddy vs. Jason, she was played by Paula Shaw. According to the actress herself at conventions, Betsy Palmer was asked to reprise her role as Pamela Voorhees for the Freddy vs. Jason film, but turned the part down when the salary offered proved to be very small.
Pamela Voorhees was born in 1930 (revealed in Friday the 13th: The Final Chapter). Her son's death worsened the symptoms of her schizophrenia, driving her over the edge to madness and murder.
At age 17 Pamela, having already had a daughter whose wereabouts are unknown became pregnant with Jason by her husband, Elias Voorhees, and on June 13th, 1948, at age 18, she gave birth to a hydrocephalic boy she named Jason, as shown in Jason Goes To Hell: The Final Friday. Because of his deformity and her antisocial personality, Pamela never sent Jason to school and was the only one who knew him as seen in Friday the 13th Part 2.
Some time before Jason's death, Pamela got a job as a cook at Camp Crystal Lake. During her working hours, she would bring Jason to camp, presumably because she had no one to watch him while she worked. Jason tried to befriend the other children and engage in camp activities, but the children viewed him as a freak because of his deformity. They also pushed him into the water and made fun of him due to his inability to swim.
On a fateful day in 1957, Jason had enough of the teasing and ridiculing that the children gave him. He snuck out of his cabin late at night to "prove" he could swim and waded into the water. The counselors weren't watching him, as they were at a party in one of the adult cabins, drinking, smoking dope, and having sex. Jason Voorhees was never recovered from the lake and presumably drowned.
After her son's "death", Pamela began hearing voices telling her to kill. In 1958, a year after Jason's death, Pamela brutally murdered two counselors whom she felt were responsible for her young son's death. Camp Crystal Lake was closed after the murders and was given the nickname "Camp Blood" by local residents. When the owner tried to re-open the camp again in 1962, Pamela returned, poisoned the water, and set several fires. The camp was shut down once again and did not reopen until 1979. Pamela lived in a house which bordered the Camp Crystal Lake property; presumably this allowed her to keep a watchful eye to make sure the camp remained unattended.
On June 13, 1979, the new owner of Camp Crystal Lake, Steve Christy (who presumably inherited the land from his family, as it is later revealed the Christy family owned the camp property before the murders occurred and that Pamela was in their employ), and seven young counselors returned to the deadly camp grounds to prepare it for reopening, even after several ominous warnings of a "death curse" by the local residents. Enraged, Pamela went on a savage killing spree, killing Christy and six of the camp counselors.
The survivor, Alice Hardy, discovered two of the murdered victims and later met Pamela. Pamela told Alice about Jason's death, during which she flew into a terrifying rage and attacked Alice and chased her around the camp, saying "Kill her, mommy," mimicking Jason's voice. During the final attack, Alice decapitated Pamela with a machete that Pamela attempted to kill her with.
Two months later, Jason (who had survived his supposed drowning and had been living in the woods as a hermit) carried out his own revenge by killing Alice. Jason had kept his mother's severed head, first to frighten the girl (by placing it in her refrigerator) and then took both her corpse and the head to his shack in the woods. In it, he built something of a shrine to his mother and subsequently placed some victims there. After he is hit in the shoulder with a machete and left for dead at the end of his first killing spree, he leaves his crudely made home and his mother.
Pamela Voorhees was initially laid to rest in a run down cemetery. When Jason was killed by Tommy Jarvis a few years later, it's presumed that she was relocated (now with her head) to Eternal Peace Cemetery along with her son in accordance with arrangements made by Elias Voorhees, the father of Pamela's son.
 Life after death
Although Pamela does not share her son's unique immortality, she does reappear in later films. The character is seen again in the climax of Friday the 13th Part 2 in which Betsy Palmer reprises her role when Jason sees his mother talking to him while in reality it is one of Jason's potential victims trying to fool him. She is seen again in Friday the 13th Part 3 when lone survivor Chris Higgins has a nightmare that ends with Pamela's corpse (played by the Second Assistant Director, Marilyn Poucher), wearing her blue sweater with head attached, reaching up from the lake to pull her under. She is seen again in Freddy vs. Jason (played this time by Paula Shaw), seen in Hell commanding her son to kill the children of Elm Street; however, it turns out that it is actually Freddy Krueger masquerading as Pamela in order to manipulate Jason for his own needs.
Reportedly, the character was to make another appearance for Jason Goes To Hell: The Final Friday in a flashback sequence which would show Pamela experimented in the occult, but was dropped when Palmer turned it down. Jason X was originally meant to feature an appearance of Pamela in the holographic projection of Camp Crystal Lake Jason is distracted by in the film; the scene was to feature Jason attacking the Pamela projection unprovoked, symbolizing how he had become completely evil.
She also made a brief cameo in the Friday the 13th video game for the NES. Her decapitated head is a very powerful mini-boss in a hidden cave. It floats around after lifting itself off a pedestal surrounded by candles, reminiscent of the second movie. Just as Jason has to be defeated three times to complete the game, Pamela can also be fought three times, and each defeat earns the player a unique item -- a powerful weapon (usually a machete or axe), then her sweater, and finally the pitchfork, which is considered to be the most powerful weapon in the game.
In the un-canon Jason vs. Leatherface, Pamela is renamed "Doris" for unexplained reasons and appears in two flashbacks, one in the first issue and another in the second; in the first flashback, brought on by Jason being asked his name by Cook, she appears, face obscured, encouraging a young Jason as he writes his name on a chalkboard.  The second flashback, caused by Jason seeing Hitchhiker abuse his younger brother Leatherface, has her violently killing Elias Voorhees with a machete while he attempts to beat Jason. 
In Eric Morse's "Camp Crystal Lake" novels, an alternate version of what happens after the ninth film, the severed head of Pamela Voorhees is a major player in the first book. There, the hunter Joe Travers finds an unmarked gravestone in the forest and digs nearby to find a wet cardboard box containing the still living head, having been reanimated by Jason's cursed mask. Pamela then gives him directions to the location of Jason's buried hockey mask, which he digs up and puts on, thus becoming possessed by Jason in the process.  Carly, the novel's heroine, later discovers the head still in its box, still in the grave. During the final showdown with Pamela and the hunter, she destroys the head with a shotgun blast, thus ending Pamela's life forever. She later reappears in the fifth book (available only as an online novel) in several flashbacks, detailing how Jason was brought back for the first time after his original death as a child.  The cover of Friday the 13th: Pamela's Tale #1. The cover of Friday the 13th: Pamela's Tale #1.
A two issue comic miniseries, entitled Friday the 13th: Pamela's Tale, detailing much of Pamela's history was printed by Wildstorm; the comics take place before the events of Friday the 13th and feature Pamela picking up Annie, one of the Camp Crystal Lake councellors indirectly responsible for Jason's supposed death in Crystal Lake, and recounting to her her past, revealing Elias abused her while she was pregnant, which caused Jason to be born deformed. Driven by what she believed to be the unborn Jason's voice, Pamela killed Elias with an axe, blew up her trailer and dumped her husbands body in Crystal Lake. After telling Annie her story, Pamela promptly murders her.