What Are Mainframes?
The term mainframe is a term that is becoming obsolete. Currently, if you have to connect to your company's mainframe, you probably do so through a special program. Once you start this special program and enter you username and password to the system, you are granted access to its command driven interface. Mainframes are older systems that are controlled through a CLI, or command line interface, instead of a GUI, or graphical user interface. The reason for these older systems still in place is because the information they hold is vitally important, large in size, and very costly to migrate to a newer system.
Typically the people that still have mainframes are large organizations, government entities, or universities. More specifically, mainframes are still common in larger banks. Instead of spending the millions of dollars replacing these systems, organizations are being more cost effective by slowly phasing them out. This is done when a new system is put in place and the older system is moved over piece by piece. The other way is naturally phasing out the older system by allowing time to outdate the information stored on the systems. Some information after a certain number of years can be erased or destroyed, for example tax information. When a migration is required, it usually means the data must be maintained indefinitely and disposing the data isn't an option.
As to the physicality of mainframes, these are usually huge servers in a server room. They have to be in temperature controlled rooms because of the heat these systems put out. Also, the technology of these older systems might be out of date. Sometimes you'll see, because of the large amount of information, the data had to of been stored on tape drives. Tape drives were the only storage technology of the time that could hold that amount of information.
Today these systems are replaced by more efficient enterprise servers. These servers are equipped with better processors, hard drives, and memory. They also operate on less electricity and put off less heat. Their cooling systems are also more advanced. Organizations are more often liquid cooling their server rooms. Some server rooms are so advanced that the company will actually build a water tower for the simple purpose of cooling their servers. Then comes the need to recover the metals in the water by the process of electrowinning or electroplating the water. As you can see, these systems can become very advanced. They are usually designed to handle millions of requests a minute. Just as important as the computer systems is the network infrastructure they rely on. Being able to design, maintain, and repair these systems is a full-time job. You will see continued growth in this area as cloud computing continues to grow.