According to wikipedia

Reseller hosting is a form of web hosting wherein the account owner has the ability to use his/her allotted hard drive space and bandwidth to host websites on behalf of third parties. The reseller purchases the host’s services wholesale and then sells them to customers for a profit. A certain portion of hard drive and bandwidth is allocated to the reseller account. The reseller may rent a dedicated server from a hosting company, or resell shared hosting services. In the latter case, the reseller is simply given the permission to sell a certain amount of disk space and bandwidth to his own customers without renting a server from a web hosting company he signed for a reseller account with.

Many web hosting companies offer reseller Hosting solutions which are basically targeted at individuals or agency who wants to offer web hosting services to their clients. This helps them to retain their clients as well as earn some money. Generally agencies look for reseller hosting in their region say an agency in India will look for indian reseller hosting . Buying reseller hosting from the same region gives you the benefit of fast support but this is not always the case.

There are many things which should be kepty in mind while looking for reseller hosting services, which i will cover in my other posts.