a server is simply a program that provides resources such as information from the database or services such as mail. a client is an application that consumes the resources such as a payroll program getting data from the database on the server or an application getting services from the server. a client/server application is therefore a distributed system that shares the load between the server and the client applications. the client and server can both reside on the same machine or can be on different machines communicating via a network.
To test the client/server application, you will need to run the server program then connect to it using the client application. if it returns the requested resources or services then the application is working fine. Hope that helps.