Making sense of memory usage on Linux
Let's look at some basic commands that report on memory usage. The first that probably comes to mind is free. The free command will tell you about used and unused memory and about swap space. Physical memory is the random access storage provided by the RAM modules plugged into your motherboard. Swap is some portion of space on your hard drive that is used as if it is an extension of your physical memory.
The first line of the free command's output contains the column headings. The second, labeled Mem: displays information on how physical memory is being used. The -m option displays the information in terms of megabytes rather than kilobytes (the default).