Samus Aran is an orphan. Her parents were part of an earth colony called K-2L (not sure if that was the name of the colony or the planet the colony was located), where they were in contact with the Chozo. The Chozo tribe saw something unique in Samus while doing their exchanges with K2L's ambassandors, when Space Pirates did a raid on the planet (not sure of the reason, but something about the colony mining fuel crystals or something).
They wiped out the colony, leaving Samus Aran an orphan. She barely survived and was taken by a few Chozo people. The Chozo seem to have something in their blood that enhanced Samus's human potential. After recovering, they helped her hone her athletic abilities and she was given the Chozo Power Suit, a modular suit which has an energy-based life-support and shielding system. The Chozo tribe is an interstellar race and thus, their artifacts are located just about everywhere, and are instantly accessible to Samus because of her suit and her blood. Samus can interact with Chozo objects even without her suit as a result.
The Chozo have prognosticated her arrival in old scriptures. The hero, the savior, the defender, the guardian (depending on which game you play). They've laid out countless artifacts just for her, it seems.
Oh here's a translated version of the Metroid Manga. It should explain a lot of backstory on Samus. It's in English!
http://www.mechadrake.com/metroidmanga.html