A development package is a package that is not needed to actually use software, but to build programs depending on it. For example if you build KDE, you need the xorg development package that controls graphical routines. If you try to build KDE without the xorg development package, you will get an error during the configure step like this:
$ ./configure [...] checking for vsnprintf... yes checking for snprintf... yes checking for X... configure: error: Can't find X includes. Please check your installation and add the correct paths!
In Red Hat and Debian based systems, the "devel" packages are usually named the exact same name as the actual dependency, but they will have a "dev" at the end. For example, the devel package for libgtk1.2 should be libgtk1.2-dev. With SUSE, the ending is "-devel". With Slackware, most packages already include development files.