"Back-end systems" loosely refers to servers, superservers, clustered systems, midrange systems, and mainframes that provide data services to users. The location of these services is often called the server farm or data center.
The server in client/server refers to the back-end systems. Client/server computing splits processing between a front-end application that runs on the client's workstation, and back-end services. Typical back-end services include database management systems (DBMSs), messaging systems (i.e., Lotus Notes and Microsoft Exchange), gateways to legacy systems such as IBM hosts, and network management systems.
Users interact with applications in front-end systems to make requests on back-end systems. The back-end systems then process the requests, searching and sorting data, serving up files, and providing other services. Back-end systems are physically close to data storage systems, so this arrangement uses the network efficiently.
Three-tier systems extend the client/server system by adding a middle system that performs some processing normally done by either the client or the server. Most important, the middle tier in mission-critical business environments holds the business logic (rules, procedures, and/or operational sequences) that is shared by all applications.
When Internet/intranet technologies are used, a Web server may exist at the middle tier. It accepts requests from clients, screens the requests, passes those requests to back-end systems, accepts the response, formats it into a Web page, and sends the Web page to the user. This system is scalable. If traffic increases, the Web server can distribute some of its workload to peer servers that are not as busy.