Backward/forward cache is the optimization which enables faster backward & forward webpages for browsers. It saves them as "in-memory cache" in browsers which stores complete page as snapshots and serves them to the user when they move forward & backward. Cool right ?

Now what is in-memory cache ?

In-memory cache stays until you close browser (Rmbr RAM ?). The other one is called as "Disk cache" which won't be destroyed (ROM).