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.

Instead of going too much technical details of DVI, here basic information is provided to easily know & use the concept. A Basic Desktop Computers need to be connected to Monitors or Display Devices to display output. Now the display output of the Desktop Computer travels to Monitor using a Cable. The Cable has two ends, one end is connected to Monitor & other end is connected to the Computer. Technically the ends joining the cable are known as ports or connectors. 

In many Desktop Computers you can find a 15 pin blue colored port to connect the cable to Monitor which is called as VGA Port. Similarily there is another type of port to connect the output of computer to Monitor with the name of DVI Port. Computers are known to work in digital format, however VGA port transfers analog signals to drive the monitor. In contrast DVI ports transfers data to Monitor in Digital, Analog or both formats. There are many types of DVI connectors available such as DVI-D, DVI-A & DVI-I. Each type of DVI connector can be identified by different pin configurations. There are various port converters (such as DVI to VGA) available out in the market as well.

When looking for a Dual Monitor Graphics Card or when buying a Monitor, you must make sure that you would be able to connect the output of the graphics card to Monitor. This is where you might require information related with DVI such as DVI Cable, DVI to VGA Converter, DVI Ports (D, A & I types), etc.