A display device is anything that will put images on a screen to see what input and actions a user would ultimately need visual confirmation.
The most common display is the default monitor. By its term means that by default any monitor should work if installed on a CPU prior to turning the power on. The screen has dials that can make the display seem blank and sometimes adjustments must be made to the display itself.
A television can be used on most modern systems.
The way to determine what display you may use is to simply find the VGA or similar type of connection on the back of the tower.
The different styles are easily defined by the number of pins (THE TINY LITTLE METAL RODS).