RPM: What Are Those -devel Packages For?

Most RPM packages only include the files you need to use that package, but not the file you need if you want to compile something that needs them. The files needed for compiling are in the -devel packages.

For example, suppose that you are trying to compile something, such as Mutt, that needs the ncurses package. While having ncurses-4.2-25.i386.rpm installed is sufficient to use applications that need ncurses, in order to compile it you'll also need to install ncurses-devel-4.2-25.i386.rpm.