Sometimes called downward compatible, backward compatible 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.
Most developers and manufacturers try to keep their products backward compatible since the Intel 80286 (first backward compatible processor) was released in 1982. However, a technology manufacturer may choose to sacrifice backward compatibility when releasing a new version of an older technology. For example, when Microsoft released the Xbox One gaming console which succeeded the Xbox 360, the Xbox One was initially not backward-compatible with any of the Xbox 360's games.
Should I say "backward compatible" or "backwards compatible"?
You should be using "backward compatible" or "backward compatibility" in all your writing. In other words, it should always be "backward" and not "backwards" when used in this context.