The most noticeable change is the site's new theme which is called Moodle Boost. It has a more modern look with extra screen real-estate in the middle column. It also scales well for mobile devices replacing the need for the Moodle app (although the app is still available).
Old course content will not be automatically transferred to the new site. You will need to backup and restore your course content from the old to the new site. The following video contains instructions on how to complete this process. Please note, that you do not need to do this if creating a course from scratch.