You're going to have to explain a bit more about what you are looking for.
The source code for Moodle is all looked after using the Git version control system.
If, you are talking about the front end (and particularly assignments, given that you are posting in the assignments forum), then there is a brand new feature in Moodle 2.5 (launching next week) that keeps a history of submissions and grades. Once a teacher has marked an assignment they can open a new submission, with the current submission + grade being stored in a history that is displayed to both teachers and students (see the settings documentation here: http://docs.moodle.org/25/en/Assignment_settings#Attempts_reopened or a walkthrough of the new feature here: http://www.synergy-learning.com/blog/moodle/moodle-2-5-improvements-assignment-resubmissions/ ).