What is the actual meaning of ‘Cache‘ in System Design?

Cache is a k-v structured thing.
and it store results that can answer the query we may request later.

what kind of DB support cache?
Memcached: Not support 数据持久化(because it store in memory, that’s why it called Mem(ory)-cache)
Redis:support 数据持久化

一些常见的误区：
cache可以存在任何地方 内存 硬盘 CPU…
cache不一定是指的服务器的缓存 前端后端客户端浏览器都可能有cache