Some years ago, the term Xposed Framework was not present in the custom ROM and developer scene. Now it seems to be everywhere.
Xposed is a framework for modules that are able to modify the behaviour and looks of the Android operating system and apps. This means, that you are able to install modules which add custom ROM like features to the Android OS running on your device. These modules include both visual- and performance improvements.
This description (quoted from trendblog.net) doesn’t really say anything, because with root priviliges (Xposed seems to require root), users could just as well alter the system in any way they like, so they wouldn’t need Xposed.
What is this Xposed Framework, how does it work, why would I want it and why is it so important nowadays?