While free displays only memory-related information, the top command does a little bit of everything. CPU utilization, process statistics, memory utilization — top monitors it all. In addition, unlike the free command, top’s default behavior is to run continuously; there is no need to use the watch command.
The display is divided into two sections. The top section contains information related to overall system status — uptime, load average, process counts, CPU status, and utilization statistics for both memory and swap space. The lower section displays process-level statistics. It is possible to change what is displayed while top is running. For example, top by default displays both idle and non-idle processes. To display only non-idle processes, press [i]; a second press returns to the default display mode.
Although top appears like a simple display-only program, this is not the case. That is because top uses single character commands to perform various operations. For example, if you are logged in as root, it is possible to change the priority and even kill any process on your system. Therefore, until you have reviewed top’s help screen (type [?] to display it), it is safest to only type [q] (which exits top).