Basically, there are several kinds of dedicated server. The most two common are unmanaged and managed. Unmanaged servers are those you have to take care and manage the server, security patches, being responsibility when the server is compromised. Managed server are someone else taking care of the server, like those guys in the datacenter, they do server patching, maybe optimizing and tuning if it's needed.
Dedicated server normally comes more expensive than a reseller packages, in return you will get much more bandwidth than a reseller package.
Reseller package is a kind of hosting that is offered by a dedicated server renter (or colocater) who seperate down the system specs and resell it. You don't have to take care the server, someone else does. You're paying an amount of money every month to get the access to create and reseller the services to the end consumer.