Client and Customer

In business, the terms client and customer have become interchangeable. The use of the terms also is tricky.

In the rules of usage, however, although the terms are closely related in meaning, they are not interchangeable.

"Client" refers to a person who pays for the advice or services of a professional person.

For example: They are both clients of Atty. Reyes.

"Customer," on the other hand, refers to a person who purchases goods from a shop, etc.

For example: Many customers are complaining about high prices and poor service.

Now, there is also an instance that the term "client" refers to "customer." In this case, "client" is used in the sense of "customer" by shops that regard it as a more superior word.

For example: The clients of an exclusive dress boutique were treated to a dinner dance.

Source:
Webster's Universal Spelling Grammar & Usage