Spider-Woman (or Spider-Gwen outside of stories) is an alternate universe version of Gwen Stacy. She hails from Earth 65, a universe where (among other changes) Gwen was bitten by the radioactive spider and received Spider-Powers instead of Peter Parker, who became their world's version of the Lizard. Gwen was able to stop his rampage, but Peter died in the process. As a result of his death, she was inspired to take the identity of a masked crime-fighter.
Using web-shooters that she acquired from her world's retired Wasp, Gwen became a vigilante who helped fight crime while as Gwen Stacy working as the drummer for her band The Mary Janes (lead by Mary Jane Watson). As Spider-Woman her nemesis became her universe's Matt Murdock, an evil blind lawyer/assassin who usurped Wilson Fisk to become the new Kingpin of Crime. She also fought against her universe's Vulture, Kraven the Hunter, Harry Osborn (her childhood friend who went insane after Peter's death, became a rogue SHIELD agent and then used his wealth to make Green Goblin tech, Hobgoblin androids and use a modified Lizard serum for powers), and the Punisher (a police-captain with a military-past who was ordered by Mayor Jameson to defeat Spider-Gwen). She also had a run-in with her version of the Venom Symbiote.
Recently, Spider-Gwen has left her universe for brief periods of time in order to visit the main Marvel Universe (Earth-616). While there, she has aided Spider-Man among other heroes, and eventually formed a romantic relationship with Miles Morales.