A cache is a storage layer that temporarily holds frequently accessed data to speed up retrieval times. It is commonly used in computer systems, including CPUs, web browsers, and databases. By keeping copies of data closer to where it is needed, caches reduce the time it takes to access information, improving overall performance. Caches can be found in various forms, such as CPU caches, which store instructions and data for quick access, or disk caches, which hold data from hard drives. The effectiveness of a cache depends on its size and the algorithms used to manage the stored data, ensuring that the most relevant information is readily available.