Think of a reseller as a retailer of domain names and server space. A reseller sets up an account with an ISP wholesaler and sells those services as if they were the reseller's own. In reality, the reseller is simply a middleman who sets his own pricing for products and services.

I advise against using resellers for multiple reasons. You can buy directly from wholesalers, who offer the best pricing and customer service. If you use a reseller, he is your liaison to your domain registrar / server company. If a scenario occurs where your reseller is unreachable, the wholesaler will not help you unless you jump through multiple burning hoops suspended over a pool of piranhas. I have seen reseller delinquency happen many times. It is to be avoided.

If you're looking for a good ISP go with Hostgator.com. If you're in the market for a good domain registrar go with Domain.com.