I was wondering: Are mangas and light novels practically the same thing? Or there are major differences between these two types of books/reading materials?

nope. light novels are kinda like novels for younger readers (which could include teens or young adults). they have plenty of artwork in them though, maybe a picture per every few chapters, often done anime-style in color. not comic book style artwork with word bubbles. subject matter is more pop-fiction than your average novel and anime-like. it's kind of like reading a professional anime fan fiction. otherwise, they're just like a regular novel. they're popular source material for manga, anime, and games. i have Slayers and Vampire Hunter D light novels.