Reem is different from other static site generators in the amount of control developers are allowed during the build phase. The source directory is expanded into a tree data structure that can be traversed and manipulated in ideally any way desired and then is written to the output directory. Plugins are added to Reem like middleware in Express and Reem seperates files into four middleware stacks, one each for posts, pages, files, and lists (folders). Reem really is quite feature rich and should not be summed up in a paragraph.
More information can be found on the wiki pages.