Alternatively referred to as a big iron computer, a mainframe is a large central computer with more memory, storage space, and processing power than a standard computer. They are typically utilized by governmental and corporate organizations for added security as well as processing large sums of data; such as consumer statistics, census data or electronic transactions. Their reliability and high stability allow these machines to run for a very long time, even decades.
In the picture above is an example of an early mainframe computer that was the size of a room and took multiple people to operate. Today, your smartphone is several time more powerful than this early room sized mainframe computer. However, is not more powerful than the quantum computers, super computers, and data centers of today.