What different versions of Linux are available?

Windows is quite good at keeping a consistent API through time. However, some older applications (especially programs that predate Windows 95) can work poorly or not at all. This is often caused by the application needing 16-bit access, or direct access to hardware. 16-bit applications run in the Windows on Windows 16-bit subsystem for compatibility, but do not always work. The 16-bit subsystem is not included at all in with 64-bit OSes. Linux suffers from some similar backward compatibility problems, but as most Linux applications are open-source, newer versions usually become freely available which fix these problems, or can be recompiled for a different architecture.