A client (a CPU, web browser, operating system) wants to access a bit of data, it believes to be in the backing store, it first checks to see if the datum can be found in the cache. If the data can be found in the cache, the client can use it and does not need to use the main memory. This is known as a cache hit. So, for example, a web browser program might check its local cache on disk to see if it has a local copy of the contents of a web page at a particular URL. In this example, the URL is the tag, and the contents of the web page is the datum.