A server is a physical computer dedicated to run services to serve the need of its clients. Depending on the service that is running it could be a file server, database server, print server, or a web server.
A server provides that service and a client gets that service. A client application requests some services from another application that acts as a server. When we access a website, we get content on our screen served by a server.
Our emails are also there on some servers and when we provide username and password, the server verifies credentials and serves our email records. An email user, in this case, is a client.
A client is a process that accesses a service provided by a server. For example, to check the email we use a web browser as a client. The client provides a user interface to carry out actions like giving username and passwords in the network. It forwards requests to the server which in return provides the required service.
It is important to know whether the client is hardware or software. In general, a client is a hardware where a cell phone, laptop, and desktop computers are acting as clients but in particular, the software running on that hardware is the process that makes it a client.