No matter how much RAM your mac is pushing, it's inevitable that your computer will eventually hang; those times when no matter how many applications you quit, your computer still seems to working in slow motion. By way of review, the simple way to quit hanging applications is to use force quit (command-option-esc) and a window listing all of your running applications will appear.
Unfortunately, this method does not include invisible processes, those background applications that keep your mac running.
To see those, you'll need to launch Terminal from your Applications>Utility folder.
The Terminal uses an interface that looks a lot like the DOS prompt of yore. Terminal is the gateway to your Mac's UNIX underpinnings.
At the Terminal prompt type the word top. No caps and no period. You'll then see a list of all running applications. The first three columns from the left contain the most important information. PID or process ID is the number associated with each process. You'll need to know this number in order to quit its respective process.
Next is the COMMAND column which lists the name of the processes at work. Note that open applications are listed as processes here too - the same ones you'd see if you used the force quit method described.
The third column is %CPU, the amount of CPU power each process is taking. In most cases, a process will take no more than 30% of your computer's power. But if your machine is running slow and you see a process that's at, say, 90%, then that process might be your culprit.
To kill a process, note its associated PID number.
Next, quit out of your top search. To do this simply type the letter q. No period. Terminal will clear the page and offer you a new prompt.
Next, enter the command kill followed by the PID number you'd like to quit. For instance, kill 9178. Do not type a period afterwards. The process will quit. If you'd like to verify, simply enter top once again.
Quit Terminal and you should be back in business.