The edX Platform uses several different modulestores to store course data. Each of these modulestores is in use on edx.org.
The XMLModuleStore was the first modulestore used for the edX Platform.
XMLModuleStore uses a file system that stores XML-based courses. When the LMS server starts, XMLModuleStore loads every block for every course into memory.
XMLModuleStore is read-only and does not enable users to change a course without restarting the server.
DraftModuleStore was the next generation modulestore and provides greater scalability by allowing random access to course blocks and loading blocks on requests.
DraftModuleStore allows editing of courses without restarting the server.
In addition, DraftModuleStore stores a draft version of some types of blocks.