Resellers are just that, they basically allow you to utilize your own "Admin" control panel where *you* can add/remove/delete/edit domains/accounts at your discretion. You decide the amount of space they get as well as the amount of bandwidth; as well as other such resources.
Semi-Dedicated usually provide root access, whereas resellers do not. Semi-Dedicated is usually for those that need the power of a dedicated, although cannot afford a $200/month+ server on their own (we do not yet offer semi-dedicated).
I do have plans to offer dedicated servers soon enough, just need to get some things cleared up before we move any further

.