Backward compatibility for software applications, what does this mean ?

Backward compatibility can be defined as the quality or ability of a device to work well with input generated by a device of older technology. For example if the latest version of a music player can still play music of old formats and types, the music player is said to be backward or downward compatible. The best examples of backward compatibility are given by communication protocols. Forward compatibility is just the opposite of backward compatibility.
In the context of programming languages, a programming language is said to be backward compatible if it’s compiler of version “a” is able to read and execute the source code or programs written in the older version of the same language compiler i.e., in the version “a – 1”. A technology or an IT product can be called a backward compatible device if it properly and equally replaces the older device of the same kind. Even a particular kind of data format can be stated as backward compatible under the condition that the program or message written in that format is still valid under the improved version of the data. For example, the newest version of Microsoft Word should be able to read documents created by previous (could be many years previous) versions of Word.
Backward compatibility can be looked upon as a relationship between two devices having similar attributes. In a layman’s language we can say that a device is called backward compatible if it exhibits all the functionalities of the older device. In the relation of backward compatibility the new or modern version device is said to have inherited all the attributes of the older one. If it does not have those qualities, it cannot be called as a backward compatible device. There are two types of backward compatibility. They have been discussed below:
Binary compatibility or level- one compatibility: It can be defined as the ability of a program to work well directly with the new version of the compiler of the language in which it has been written without its recompilation or modification.
Source compatibility or second level compatibility: it can be defined as the ability of the program to work well and effectively with the new version of the compiler of the language in which it has been written, but, with recompilation of the source code and also with condition that the source code should not be changed.

Many programs and devices use various technologies to achieve backward compatibility. “Emulation” is one such technology. In emulation, the platform of the older software is simulated into the platform of newer software and thus providing backward compatibility. There are so many examples of backward compatibility available. A few have been listed below:
Blu ray disc players can play CDs and DVDs.
Newer video game consoles are able to support games which were created for preceding video game consoles like Atari 7800 and Atari 2600, game boy advance and game boy systems, Nintendo DS and Nintendo DS lite, Nintendo 3DS and Nintendo Ds and Nintendo DSi, play station 2 and play station 3 and play station, PSP and Psone, PS vita and PSP, Xbox 360 and Xbox, wii and Nintendo etc.
Microsoft windows are backward compatible with shims, where the software of the newer version is tweaked specifically to work with already released products. For example, when Microsoft would have released Windows 7, they would have tested existing software applications that works with Windows XP and Vista and made changes inside Windows 7 to ensure that it works with these software as well.
Mac OS X Intel 10.4 versions have been made backward compatible with Mac OS X 10.6 Intel versions through Rosetta which is a binary translation application.
Microsoft word 2000 is backward compatible with Microsoft word 97 and Microsoft 2008 is backward compatible with Microsoft 2007. The other applications of Microsoft office follow the same backward compatibility pattern.
Even some cameras show backward compatibility like Nikon f mount and Nikon DSLRs, canon EF mount and canon APS-Hs etc.