In my System Monitor panel applet in Linux Mint 17.3 Cinnamon, it shows me memory usage of used memory and cached memory combined, whereas in the system monitor application, it just shows used memory, so the system monitor tends to show me much higher memory usage. What exactly is cached memory, and is it something to be worried about if a lot of memory is being used by it?
Cached memory is memory that Linux uses for disk caching. However, this doesn't count as "used" memory, since it will be freed when applications require it. Hence you don't have to worry if a large amount is being used.
Here's a nice page about memory usage and free, used, and cached RAM: http://www.linuxatemyram.com/