When talking about VPS (vertical privet server) root access is the highest level that someone can have, unlike an admin account where they more have day to day access that more deals with helping users fixed small issues and a user account has limited access on what they can and can not do.
Root: Root has access to all files and folders on the VPS, can access almost all files and folders that admin and users do not have access to. Making edits to some files can cause a whole VPS to go offline affecting everyone. Root access should only be accessed when user and admin cannot fix the issue.
Admin: As said above admin access more has day to day running of the VPS and can access mostly all users files and folders like the root level does but admins cannot access any files and folders for the most part that can take a whole server down. Still editing some files can cause a website go to offline and if you are editing the main domain name that has the name servers i.e. ns1.wqyu.com, editing some files in wqyu.com can cause the server to go offline.
Users: Users have access only to their account and when in their account they can access all of their files and folder for the running of day to day operations but users cannot access any files and folders that can cause a server to go offline for the most part, unless they some how can access files and folders they should not have access too.
Each level root, admin, and user has different permissions so it is defiantly recommend that you do most if not all of the uploading of files and folders for the general running of day to day operations using either admin or a user level as the root access may cause issues when trying to access your site as a guest.