Sometimes called downward compatibility, backward compatibility is a term used to describe software or hardware that is compatible with previous versions of software or operating systems. Without backward compatibility, a program that works with one computer processor or operating system would stop working with the new version. For example, almost all software running on a Windows 7 computer would work after you upgrade to Windows 10.
In general, developers and manufacturers always try to keep all of their products backward compatible and have been doing so since the Intel 80286 (first backward compatible processor) was released in 1982. However, it may be necessary for companies to sacrifice backward compatibility to take advantage of new technologies. For example, when Microsoft moved from the Xbox 360 console to the Xbox One, to make use of newer technologies the Xbox One was initially not backward with any of the previous consoles games.
Should I say "backward compatible" or "backwards compatible"?
You should be using "backward compatible" or "backward compatibility" in all of your writing. In other words, it should always be "backward" and not "backwards" when used in this context.