Spider-man (aka Peter Parker)'s first true love, most famous for being one of the few comic book characters to die permanently.
The couple first met in Amazing Spider-man #31 in 1965 while both were science majors at Empire State University. Although Peter was at first too busy with his new Spider-Man role to pay her much attention, eventually a romance sparked off between the two.
Gwen (real first name Gwendolyn) was your typical sweet bubbly All-American girl, blonde hair, blue eyes, and always a friend to Peter. It's kind of sad there's not more to say about her, but her wholesomeness and simplicity is part of what makes her story arc so shocking.
In Amazing Spider-man #90, Spider-man accidentally killed Gwen's father, police officer George Stacy, while fighting with Dr. Octopus. Despite the accident, Gwen blamed Spider-man for her father's death, causing a major strain between her and Peter (as she didn't know his secret identity.)
After taking a year off in Europe to grieve, Gwen returned and things resumed as normal, until Amazing Spider-man #121, released in June 1973. In that issue, Harry Osborn, Peter and Gwen's friend, is suffering from the effects of drug abuse. When Peter and Gwen go to visit him, his father Norman Osborn (previously the Green Goblin but "cured" via amnesia) throws them out, blaming them for Harry's problem. After learning he is going broke, Norman has a breakdown, once again donning the Green Goblin costume to kill Spider-man - who he knows is Peter Parker.
To do so, he kidnaps Gwen Stacy and lures Spider-man to a battle at the Brooklyn Bridge. Spider-man ultimately overpowers the Green Goblin, but at the last moment, the Goblin sends Gwen hurtling over the edge of the bridge to her doom. Spidey's instincts kick in, he shoots his web out to grab her, stopping her fall with a sudden snap, but when he pulls her back in -
It became "The Night Gwen Stacy Died".
For fans, this was unthinkable: a major character dying, and a good one at that. For many it marked the end of the Silver Age of Comics, bringing about the grittier and distinctly grimmer Bronze Age. For the writers, though, it was merely an inevitable conclusion. Gwen and Peter couldn't get married - it wouldn't do to have Marvel tie down their hottest property to a domestic situation - and Gwen had just about run her course in terms of the whole "Peter / Spider-man" mystery.
Gwen's death only had a few lasting consequences on the Spider-man timeline. The most prominent one is that Mary Jane Watson, formerly just a party girl and occasional character, became much closer to Peter Parker in the wake of Gwen's death. The two, of course, eventually got married and Mary Jane is an integral part of Spider-man's life. On a secondary note, The Punisher was introduced as a hired assassin to find Gwen's killer, whom he believed to be Spider-man.
Critics often point to Gwen's death as the first in the famous line of victims of the misogynistic Women in Refrigerators syndrome, although in the writers' defense, her character's importance to the series transcends many of the other wives and girlfriends later afflicted by this business.
Perhaps the most controversial (certainly the most argued) aspect of Gwen's death is whether Spider-man's attempt to stop her actually killed her. The panel showing her being "saved" has a small "snap" sound effect down by her neck, seemingly indicating her cause of death. However, later issues of the same comic removed the effect (most likely to increase the ambiguity of the argument) and it is never explicitly stated what caused her death.
Despite remaining permanently dead, Gwen has popped up as a player in a couple of Spider-man story arc backstories: in the Clone Saga, a clone of Gwen Stacy shows up to complicate matters with Peter and Mary Jane (and to be used as a decoy to lure Spider-man into a trap), and in Sins Past it was revealed that while in Europe she gave birth to two twins with Norman Osborn.
Bryce Dallas Howard plays Gwen Stacy in Spider-Man 3, acting as girlfriend of Eddie Brock and a possible love interest for Peter Parker.