uptime

   This command will show current number of users, and the time elapsed since
   the computer last rebooted.  It also indicates the load average over given
   periods of time.  Three numbers will be shown, such as:

        Load average:  0.03, 0.05, 0.01

   These numbers indicate the amount of activity on the system over the last
   1, 5, and 15 minute intervals, on average.  Lower numbers indicate less
   activity.

man

   Manual pages are provided for most commands on a UNIX system.  Information
   on the smallest details of a command can be found with man.  Uses:

        man uptime               Show information on the 'uptime' command
        man -k users             Find all commands relating to the word 'users'

   There are also manual pages for all common C/C++ functions, among other
   things.

ps

   Used to see what processes the machine is running.  Uses:

        ps                       Shows processes being run by your account
        ps -u <user>             Shows processes being run by <user>
        ps -aef                  Lists all processes (output will be long)

top

   Shows processes using the most CPU time.  Can also sort by memory use.
   Updates regularly.