> Could you guys explain to me what Haskell would be mostly useful for. For instance, can it be used to make a web app backend or a GUI app?

Yes, and yes.

> Also, can you recommend a good book or resource that uses real world examples to teach Haskell?

The obvious thing to recommend here is Real World Haskell [0], which directly addresses some of the areas you raise.

Also, Write Yourself a Scheme in 48 Hours [1] is more in-depth and real-world than most tutorials (writing a Scheme interpreter isn't exactly a common real-world application, but its more real-world scale than most tutorials address, and it uses a lot of things that are of concern in many real-world apps.)

[0] http://book.realworldhaskell.org/read/

[1] http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_H...