As the article mentions, at one point Marvel was really broke and simply started licensing out the rights of characters to different studios, here's a breakdown of what studios own which characters.

http://screenrant.com/marvel-comics-movies-characters-carl-6...

Because they sold the rights of the X-men to Fox they can't use that character in their own movies. Same with Spiderman to Sony until recently (last month) where Sony is going to let Marvel use Spiderman in the Marvel Cinematic Universe and also produce future Spiderman movies (Sony was simply constantly rebooting Spiderman because of his limited stories and hasn't been able to expand the character and story too much with other characters).

Marvel has started getting back some of these characters, mostly by defaulting it back (studios are forced to release a movie every X years or the character goes back).