Mainframes are large, powerful computers that run critical enterprise applications for banking, finance, insurance, and telecommunications. They operate on IBM's Z/OS or OS/390 operating systems and use programming languages like COBOL, PL/I, and Assembler. Mainframes provide advantages like high processing speeds, security, reliability, and the ability to support many concurrent users. While 80% of mainframe projects now are maintenance, over half of organizations still rely on major applications hosted on mainframes.