What's the difference between "client" and "customer"?

A client is the one to be served, economically or not, the economical one being a customer. A customer (purchaser) is not necessarily a client, when product(s) rather than services are offered.

A consumer is the one who uses products or services, paid or not. So, a consumer is not necessarily a customer. Children at toy shops are usually consumers, their mothers being customers.

Client to me has an element of "ongoing relationship with the seller" in its meaning: if you go to the store to buy a box of matches, you're just a customer, but if you always go to that store because you know you'll get good service and good prices, you're a client. (However, the dictionary does list "customer" as one of the meanings of "client", so they are very close in meaning if not identical.)

I would say that someone who pays a monthly fee in order to have continued access to an online service could be called a client.

Some suggestions for your terms:

• malicious bot = Abuser
• entity using programming tools = accessor
• non-using public = public
• registered but not paying = user
• payer = client

Embrace the jargon

Obviously, you might confuse the reader if you mix jargon from different disciplines in the same work. So if you're a lawyer or paralegal I could understand why you'd want to use words like "client" and "consumer" only in the sense that a lawyer would expect.

But in most discussions about the business and software around online services (you mentioned "free," "fee-based," "bots," and "programming tools") you're not going to be able to escape the jargon that was invented for those subjects. In that jargon:

A service is something you offer that (you hope) has value to your customers.

A server is the hardware that provides that service. The words "server" and "service" also used of the software running on that hardware, with "service" being more common in Microsoft shops and "server" more common elsewhere.

A user, consumer, subscriber, or customer is a person you're doing business with or providing a service to. When the person paying for something isn't the same as the person you're delivering it to, reach for more specific terms to make the business distinction. For example, Google provides a free service to the public and makes money on advertising; they'd use a word like "consumer" for people in one group and "advertiser" or "business partner" for the other.

A client is any software that interacts with the server. The word is also used to describe anything on the user's side of the interaction, e.g. "client PC."

A bot is a client accessing a service for some reason other than immediately presenting something to a person. Bots specific to the web are also called spiders. Bots are not necessarily malicious; for example Google has lots of unattended programs crawling the web and making an index entry for each word they see. Instead of "malicious," consider a word that describes what you're opposed to, such as unauthorized or unacceptable, and define what constitutes authorized or acceptable use.

For your other examples I'd use user for a member of the general public, member or (free) subscriber for someone who registered, and paid or premium subscriber for people forking over money.

A client is generally someone purchasing a professional service, contrasted with a customer who might be buying a box of matches. Perhaps a client is a bit more genteel, and possibly richer. (Myself, I call them both customers because I call a spade a bloody shovel.)

(See http://dictionary.reference.com/browse/customer, http://dictionary.reference.com/browse/client.)

A client is someone who engages the services of a professional. For example, lawyers, plumbers, freelance writers, accountants, and web designers often work for clients.

A customer buys goods or services from a business (rather than an individual or group of professionals).

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