top will continuously show your machine's load and which programs are taking what percentage of resources, in real time.
Main commands in top:
s will change the delay (in seconds) between two refreshes of top. Split-seconds aren't allowed but for quick refresh you can use zero. o followed by one of the following changes the sorting of the display by time of: TIME, RES, SIZE, CPU, PRI k will send signals such as kill to specified tasks. m toggles the display between CPU and I/O mode. u show only the processes of a certain user r renice a process for altering its speed. Some renice acts are restricted to root or su mode. S toggle the display of system processes t toggle the display of top itself d change the number of time to refresh top before ending it. q (at the command line only as "top -q", restricted to root only) to renice top to -20 for extra fast refresh
Most of the commands work the same way in the command line.