Free of charge, simple to learn/use, Cross Platform C library for GUI Apps? Am I looking for Qt?

Bonus question: Can I develop with the said library/toolkit on Mac then recompile on PC/Linux?

Super Bonus Question: Link to tutorial and/or download of said library.

(RE)EDIT:

The truth is that I'm in the process of catching up on the C family (coming from web development - XHTML/PHP/MySQL) to learn iPhone development.

I do understand that C is not C++ or ObjectiveC but I want to keep the learning curve as simple as possible. Not to get too off topic, but I am also on the lookout for good starter books and websites. I've found this so far.

I'm trying to kill many birds with one stone here. I don understand that there are platform specific extensions, but I will try to avoid those for porting purposes The idea is that I want to write the code on one machine and just compile thrice. (Mac/Win/Linux) If Objective C will compile on Windows and Linux as well as OS X then that's good. If I must use C++, that's also fine.

EDIT:

Link to QT Please...