Well, actually QT-Designer is just a GUI Builder for applications based on QT. QT itself is a quite complex library written in C++; it is mostly used for GUIs but provides a lot of other things, too. Have a look at its homepage for more infos:
So you won't make whole programs with QT-Designer, but only their user frontends.
KDevelop on the other hand is a full IDE, so use this (or any other IDE, there are a lot of them) to develope your applications.
Oh, and another word on QT (just to avoid misconceptions): As I said, this is only a C++ library, so if you want to learn C, then this won't be the place to go.