In very simple terms, cache is a memory block where we store preprocessed information for the application. In this context, a key-value storage, such as a map, may be a cache in the application. In Spring, cache is an interface to abstract and represent caching. A cache interface provides some methods for placing objects into a cache storage, it can retrieve from the cache storage for given key, it can update the object in the cache storage for a given key, it remove the object from the cache storage for a given key. This cache interface provides many functions to operate with cache.
We use caching in cases where a method always returns the same result for the same argument(s). This method could do anything such as calculate data on the fly, execute a database query, and request data via RMI, JMS, and a web-service, and so on. A unique key must be generated from the arguments. That's the cache key.