The basic difference between a reseller and shared hosting is flexibility of using your account for reselling your resources. Shared hosting is for one account holder where they can use it for hosting their website using shared resources with other websites.
Reseller hosting plans allows you to use your hosting account for hosting multiple users with multiple control panels at price you want to fix. Customization is allowed to help provide personalized service to their own clients. It is very much like sub-letting the apartment you are renting.