If someone were to break in your room, they would probably learn less about you than if they hacked you on the internet. Our efforts towards security have to be as intensive online, as they are offline.
The Drupal Cache API is used to store data that takes a long time to compute. Caching can either be permanent or valid only for a certain time span, and the cache can contain any type of data. To make websites faster Drupal stores web pages in a cache.