Final answer:
Large, expensive computers designed to support hundreds of users simultaneously are known as Mainframes. They have extensive storage and processing capabilities and are distinct from personal computers and supercomputers in their usage and optimizations.
Step-by-step explanation:
Large, expensive computers that support hundreds of users simultaneously are called Mainframes. These powerful systems are essential for large organizations for bulk data processing, enterprise resource planning, and transaction processing. They differ from personal computers, which are intended for individual use, and supercomputers, which are optimized for complex, computational tasks such as scientific simulations and calculations. Mainframes are known for their large storage capacities, robust processing power, and ability to handle multiple tasks at once with reliability and efficiency.