Caching is the act of keeping data in storage to allow retrieval without having to request the data from the original source, if that data will not change frequently.
A typical caching scenario is having a cached copy of a web page. That page doesn't change every five minutes, so caching it locally on your computer saves time and bandwidth for you to re-display it if you hit reload in your browser.
Cache is generally much faster than loading from disk or having a server generate a page from scratch. There are many other caches such as a database cache, or a disk cache, that speed up access for all kinds of events.
A cache server is a dedicated network server or service acting as a server that saves Web pages or other Internet content locally. By placing previously requested information in temporary storage, or cache, a cache server both speeds up access to data and reduces demand on an enterprise's bandwidth.