We all visit websites and notice sometimes it seems to load faster than usual, and sometimes it takes longer than usual, especially when you notice that the page content seems a bit different, meaning the owner updated the website. This is often due to the cache. Cache is often useful, but also often the reason of lots of issues. Website Cache is when you visit a website, and it pulls static content from the most recent other visitors instead of all the code directly from the origin server. In return, you get a faster load time. The downside is, even though you usually get an improved load time, it can sometimes display outdated content, or companies like Google can use the Cache and sell your visitor information. Cache absolutely isn't required, companies say it is so important and that every big website uses cache. Part of that is true, lots of big websites do use cache, but it is absolutely not necessary.
There are two different types of caching, browser caching and serverside caching. Browser Caching occurs on your local browsers, like your Firefox Installation or Chrome installation. Serverside caching occurs on the server that the website is hosted on. For example, if serverside caching occurred on google.com, there would be cache stored on google's servers that would be served to you. If local browser caching occurred, the cache would be pulled from your local browser and served to you. Browser caching is not recommended at all, it is known to cause issues and lots of confusion. The only type of cache that would be acceptable is serverside cache, but this needs to be cleared after every time content is changed, modified, added, removed, etc.