Seth Godin explains the difference between a client and a customer
<code>The customer buys (or doesn't buy) what you make. The client asks you to make something. The customer has the power to choose, but the client has the power to define, insist and spec. </code>
It's an important distinction to make. I've heard these terms being used pretty often with simply the meaning of the person/company that pays for a service or a product.
It seems as important to realise the difference between the two and make conscious decisions on which to focus on.