After her nine appearances in the Marvel Cinematic Universe, Natasha Romanoff, aka Black Widow is a character we know pretty well — at least as much as she'll let us know. Scarlett Johansson's character is famously kind of reticent. We know that she was trained to become an assassin as a young girl, at a facility called the Red Room alongside other Black Widows. We got a glimpse of what that experience was like for her in Avengers: Age of Ultron, when she had a flashback to her training and told Bruce Banner (Mark Ruffalo) about her involuntary hysterectomy. Bu this new movie fills in the massive gaps in her story.
Nat ultimately escaped the Red Room to become an Agent of S.H.I.E.L.D. first and, later, an Avenger. She doesn't have superpowers. In some comic book iterations, the Red Room dosed Nat with a version of the super-soldier serum, but on the big screen, she's just an international superspy.
As it turns out, she comes from a family of international superspies — a group we finally get to meet in Black Widow.