An application or utility program usually involves quite a few files. It might include libraries, data files like game scenarios or icons, configuration files, manual pages, and documentation. When you install the program, you want to make sure you have all the files you need in the right places.
You'd also like to be able to uninstall the program. When you uninstall, you want to be sure all the associated files are deleted. However, if a program you still have on the system needs those files, you want to be sure you keep them.
Finally, you'd like to be able to upgrade a program. When you upgrade, you want to delete obsolete files and add new ones, without breaking any part of the system.
The Debian package system solves these problems. It allows you to install, remove, and upgrade software packages, which are neat little bundles containing the program files and information that helps the computer manage them properly. Debian packages have filenames ending in the extension .deb, and they're available on the FTP site or on your official Debian CD-ROM.