She has a very long and convoluted origin story. In a nutshell, she gained her powers somehow when she saved the original Kree Captain Mar-Vell. She took the title "Ms. Marvel."
She lost her powers when the mutant Rogue stole them AND her memories (accidentally) in a fight with the X-Men.
I missed the part where she regained her powers because Rogue still has them. In any event, since Mar-Vell died (cancer, if I recall correctly) she assumed the mantle of "Captain Marvel."
There is a new Ms. Marvel, a mutant who can change the size of her limbs at will. She was billed as Marvel Comics' first Muslim superhero.
That's all I know. I haven't really been following the comics all that closely since I stopped commuting into Manhattan (I grabbed the comics at the train station so I had something to read during the ride).