Here is an excerpt from brilliant minds at Bold Thinking

Consumers versus customers:  In the world of the creative for the purposes of crafting compelling concepts that attract or initiate action, a consumer is anyone who uses your product or service independent of the buying relationship.  A customer on the other hand, is the person that you have an economic relationship with, the person who is engaged in the transaction. Therefore, a consumer and a customer can be the same individual. Often however, they are not (think mother customer, child consumer).  One can generalize that all customers are consumers, but not all consumers are customers.

You can read the entire entry on their website: www.boldthinking.com/ourphilosophy.html