Client or Customer

We normally use both words generically to describe whom is the purchaser, the consumer or everyone that can be considered the beneficiated by a product or service we provide and charge.

In fact there´s some difference between the terms?
I believe so, and this point starts by the source.
According to Dictionary.com these words means:

Client
1. a person or group that uses the professional advice or services of a lawyer, accountant, advertising agency, architect, etc.
2. a person who is receiving the benefits, services, etc., of a social welfare agency, a government bureau, etc.
3. a customer.
4. anyone under the patronage of another; a dependent.
5. Computers . a workstation on a network that gains access to central data files, programs, and peripheral devices through a server.
6. client state.
7. (in ancient Rome) a plebeian who lived under the patronage of a patrician.

Customer
1. a person who purchases goods or services from another; buyer; patron.
2. Informal . a person one has to deal with: a tough customer; a cool customer.


As the descriptions the main difference is on the relationship with this buyer. I mean, the right word has total dependence if you are talking about a single and temporary exchange or a long-term relationship with whom is at the other side of the desk.

A valuable relationship goes beyond to ensure the transformation a simple customer in a loyal client.

You have to take care of him.

Your products and services has to bring him the reason to stay, almost dependently.
These concepts are defined in the dictionaries: protection, patronage, dependent, etc. All establishing a context where someone is watching and taking care of something for the other. Obviously in a exchange relation directioned to clearly demonstrate that it worth for. Cent by cent.

This seems to be the goal when talking about clients:Create among social relations an environment of mutual development and perpetual growth.