Prospective clients refer to the customers whom have the ability to buy your products/services and in the same time have the willingness or the interest or the need on your products or services, you can't consider a certain group of people as a prospective clients without meeting the above conditions. For example I don't eat pork so you can't consider me as a prospective client for you if you produce it even if I have the ability to purchase it in term of money.

The phrase "potential client" is just a bit colloquial. In writing, we prefer "prospective client".

prospective = future possible

yes. "Prospective clients" or sometimes we call them "potential clients" (rarely used) refers to those people/agencies whom you expect to become your clients in near future. They are not your clients right now. But, there is reason to believe that if you follow a good marketing strategy, they can be your genuine clients in near future.

i think it's like potential , expected clients :)