A GUI is an acronym for graphical user interface. GUIs use windows (forms) to make programs. The Lazarus IDE works with Windows, Mac OS X, Linux and FreeBSD. Other OS maybe supported in the future. This makes porting applications, from one OS to another, very easy. The IDE and coding for GUI Apps are very similar to Delphi.

Free Pascal and Lazarus can be downloaded at freepascal.org

Video Tutorial: Free Pascal Tutorial 15 - Starting Windows, Mac and Linux GUI Programming