In Order to connect and control dual monitors to your computer, you would need dual monitors, appropriate wires, and dual monitor graphics card. In case you are planning to use dual monitors with your laptop and your laptop does have an extra port to connect a monitor or projector, you do not need to worry about the graphics card. However connecting an extra graphics card to your desktop’s motherboard might not help as the motherboard might disable in built graphics card when it detect an extra graphics card connected to it and you would need a Dual Monitor Graphics Card for your Desktop Computer.
Dual Monitor Graphics cards can be identified with two ports connected to the same graphics card. There are many types of ports (or connectors on the graphics card to which you can connect your monitor to) such as VGA Port, DVI Port, DisplayPort, S Video Port and many other types of ports. VGA Port is blue in color and has 3 rows of holes in it with 15 holes in it whereas DVI Port is white in color. A simple Dual Monitor Graphics card can have 3 ports i.e. One VGA Port, One DVI Port and one S Vide o Port or it can have 2 DVI Ports and one S Video ports or may be some other combination of ports.
Browse to the Hardware Category from the top of the page to know exactly what you are looking regarding hardware for setting up and using dual monitors. From Dual Monitor Software perspective, Windows does have good support for dual monitors for latest operating system users.