A Dual Monitor Card is a hardware device whose task is to drive atleast two Monitors / Display Devices at the same time. A simple Dual Monitor Card may have 1 VGA, 1 DVI and 1 S Video Port available for connection to Monitor or for TV Output.
A Dual Monitor Card can be of two types and you can select any one of them depending on your requirement as given below:-
Internal Dual Monitor Card : An internal card can be typically be connected to a Desktop Computer’s Motherboard. In case you do not have any other graphics card then after inserting the dual monitor card into the motherboard, you will be able to connect two display devices to that card. In Case you have two VGA Monitors, then you will require a DVI to VGA port in case your dual monitor card does not have 2 VGA ports. Similarly depending on which ports are there in your display device and which ports are available in your dual monitor card, you might or might not need converter. This blog does have more information about Radeon Dual Monitor Card which also has more information on dual monitor cards.
External Dual Monitor Card : An External Dual Monitor Card can be useful when you wish to connect dual monitors temporarily and need flexibility. They can also be useful when you wish to connect dual monitors to a Laptop where laptop does not have an extra port to connect an extra Monitor. You can get an external dual monitor card with USB connection from lots of online shops. The term USB Graphics Card can also be useful when searching for it. The Speed of an external Monitor Card can be slower than an internal dual monitor card.
In case you would like to read more about Dual Monitor Cards and delve deep into the topic, do have a look at the post of Dual Monitor Graphics Cards.