A mainframe is a standalone set of computing hardware, while a server is a type of data transfer system working in conjunction with one or more separate client machines. However, a mainframe can also be considered a server if it is configured as such.
Comprised of several dozens of central processing units, terminals and communications channels daisy-chained together, mainframes are centralized juggernauts of information storage and processing power capable of handling complex tasks simultaneously. They host and execute all their own applications and serve their own user terminals. Servers, on the other hand, are typically software applications running on dedicated, or shared, machines, acting as recipients of client requests, or posts, to a particular database located on a local area network or a wide area network.
Historically, “mainframe” was the name given to the office-sized computers of the 1960s, 1970s and 1980s. Before personal computers became ubiquitous, these mainframes were the most common type of computing system. Since then, the term has been reserved for the large, centralized systems of complex organizations and businesses. Mainframes are designed for massive computing power as well as reliability and scalability when handling data across multiple communication channels. In contrast, servers have historically been used in external data transfer, such as between hosts and clients communicating online.