The term Server is an overloaded term which can sometimes lead to confusion. All the uses of the term Server imply a Client-Server model wherein a Client makes a request to a Server that fulfills that request and provides a response back to the Client. The Client/Server model can be chained together so that a Server can become Client to other Servers.
A Server is either a computer program or device providing services to other computer programs or devices. The users of the service is referred to as a client. Some examples of software servers are mail servers, database servers, web servers, application servers, etc. The physical device (i.e., computer) that hosts the software servers is also referred to as a server. The physical server can be dedicated to host a single software service (i.e., mail server) or it can be used to host multiple services. Complicating the issue is that sometimes a single software service might span across multiple physical servers.