When ever I am called to perform system administration on a Linux server there are several command line tools I use to familiarise myself with the machine (like df -h to see the disks). And invariably one of the commands I will use early on is the top command.
top displays real-time live view of the running system including system summary information and a list of tasks.
By using top you can quickly establish the following:
1) Is the system overloaded?
2) Is there one particular task which is consuming lots of CPU time?
3) How much memory does the machine have and is it using large amounts of swap space?
Interaction with top is limited but you can do a few useful things like update the refresh frequency (which defaults to every 3 seconds) by pressing s and then entering the number of seconds.
You can also sort the processes according to memory usage by pressing F (note the capital), n and then ESC.
To see a summary of the help press h and to exit top press q.