Caching is the method used by various programs of keeping parts of the web at hand. Most browsers use a cache so that when you revisit a webpage later, it can be retrieved from the cache and show to you more quickly, and saving bandwidth in the process.
If you are looking at a web page that you know is stale, you can try the “Refresh” or “Reload” button on your browser to force it to retrieve the latest version from the internet, instead of the cache.
Proxy servers are a sort of dedicated cache that may be used by corporations that provide internet service to hundresds or thousands of people. A CDN is another type of cache used for the distribution of content across the internet.