The *-devel packages (usually called *-dev in Debian-based distributions) are usually all the files necessary to compile code against a given library.
For running an application using the library libfoo only the actualy shared library file (*.so.*, for example libfoo.so.1.0) are needed (plus possibly some data files and some version-specific symlinks).
When you actually want to compile a C application that uses that library you'll need the header files (*.h, for example foo.h) that describe the interface of that application as well as a version-less symlink to the shared library (*.so, for example libfoo.so -> libfoo.so.1.0). Those are usually bundled in the *-devel packages.
Sometimes the *-devel packages also include statically compiled versions of the libraries (*.a, for example libfoo.a) in case you want to build a complete stand-alone application that doesn't depend on dynamic libraries at all.
Other languages (such as Java, Python, ...) use a different way of noting the API of a library (effectively including all the necessary information in the actual library) and thus usually need no separate *-devel packages (except maybe for documentation and additional tools).
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