Clients hire a service provider to perform the service, usually with an ongoing relationship, and can generally negotiate terms. Consumers purchase services (or goods) for the provider's stated price on a case by case basis, with no expectations of a continuing relationship.

I agree with Mike. His is a good answer. I would add that consumers really just consume. Usually they buy the things they consume, but not necessarily. The reason I mention this is that the context of the sentence suggests this might be the case in whatever you are reading.