A package is a way to simplify installation and upgrade of programs. They provide support for dependencies, pre- and post-install scripts, ...to simplify the life of the administrator in charge of installing the program. Often, distributions policies (e.g. Fedora Project) specify that no interaction should be involved when installing a package (which means that the user installing the package doesn't have to answer questions before or after the installation, ...). This requirement leads to highly easy installation processes.
The program itself may be pre-compiled, or not. For instance, one would not install the Apache webserver whithout recompiling it, because it needs to be tweaked (adding the right modules, ...) but it is more convenient to install a binary version of OpenOffice rather than recompiling the entire suite...
One can easily create package by simply doing an archive of the source files. The user will have a couple of Makefile 's targets to install the binary. Unfortunately, as we mentioned before, every distribution has its own rules of how to install a program (directories tree, ...). Fighting against these conventions often lead to legacy systems on which maintenance is difficult. That is the reason why a couple of distributions use packages, so that vendor programs can easily fit in the OS by following the conventions. To reach that goal, one need however to be careful to follow the instructions of how to create the packages.
In the next sections, will will describe how to create DEB, RPM and TBZ packages.