Last night, I was preparing a presentation & while proof reading it, I realized that I was using the term "customer" & "client" quite interchangeably. A quick Google search confirmed that the same question occurred to a lot of people.
While some did not find any difference, according to WiseGeek’s quote of the American Heritage Dictionary: the main difference between a customer and a client is that a protective, ongoing relationship is formed with a client, but not necessarily with a customer.
In short, as PathFinder said on Yahoo! Answers: "[it’s] Nomenclature only, but in meaning NONE. When you buy from a store, you are a customer, when you hire a lawyer, you are his client".
Going back to my presentation, I stuck to customer since the presentation spoke about "customer friendly", "customer oriented" & "customizable" even though the audience would be clients as per the definition by American Heritage Dictionary. The only place I used client was to describe "client care" since "customer care", "customer support" or "support" do not throw up positive images any more.
I hope the clients are not that discerning during the presentation; else I would need to refer them back to this post & point out that I am still using the words interchangeably.