Lazarus is an open source, cross-platform integrated development environment (IDE) for the open source Pascal compiler, Free Pascal (FPC). For the purpose of this book, it is to be assumed that when we say Lazarus, we are talking about Lazarus in conjunction with Free Pascal. Free Pascal offers a high degree of compatibility with Delphi's Object Pascal language, which includes many of the same libraries. The Lazarus Component Library (LCL) is, in many cases, equivalent to Delphi's VCL, including versions of many of the same controls used to create applications that have a graphical user interface. Just as with Delphi, Lazarus can also be used to create console applications, dynamic link libraries, or web applications.

With all the given similarities to Delphi, it is quite obvious that Lazarus along with Free Pascal can make a suitable open source replacement for Delphi. But, Lazarus is not limited to this. Lazarus can be used to develop Free Pascal applications that can be compiled and run on Linux-based smartphones. It is also possible to develop web applications with Lazarus using additional packages, such as Fpweb, FreeSpider, and Raudus. Compiled libraries can be created, which can be accessed by other programming languages, such as Python or C++.

Lazarus' greatest advantage is that it allows programmers to create cross-platform applications. Lazarus' technique for creating cross-platform applications is referred to as write once compile anywhere. This alludes to the fact that you can write applications on one platform, such as Linux, and compile it on different platforms, such as Apple's OS X or Microsoft's Windows operating systems. Currently, Lazarus supports Windows, WinCE, Mac OS X, iOS, Linux, and Android operating systems. In this book, we will focus on Linux and Windows as these are the largest current user bases in the Lazarus community.