A reseller, is merely a merchant who sells the services provided by the actual provider. This is not to say that the reseller does or cannot offer their own services, like web design services for example, in addition to the hosting services. Resellers do not own the servers, or manage the network, or any of the truly technical stuff that the actual provider does. When you call a reseller for support issues, most of the technical problems are relayed to the actual host. In essence, the reseller functions in a similar fashion as a retailer, reselling goods provided by someone else at a profit.
An actual web host provider is selling services from servers that it owns and maintains. Typically, they manage the machines, networks, DNS, and the actual uplinks to the Internet backbone. When you call a true hosting provider for support, they handle it directly.
If you have a high traffic website, or if you wish to purchase a dedicated or co-location account, it may be worth your while to ensure that you are working with an actual hosting provider. This ensures that your account can receive the highest level of attention possible, since all agreements and issues are addressed directly with the people who are responsible for the work, instead of through intermediaries who may or may not communicate your needs effectively.
Now, this is not always the case. Some hosting resellers are well equipped to handle even the most demanding of needs. But overall, for those with an overpowering need for the highest quality relationship, working directly with a hosting provider is your best course of action.