The average user may only use one (1) computer monitor when they use a computer. These days it is quite easy to use more than 1 monitor and increase your productivity.
In order to use more than one monitor you will need the proper hardware. Many laptops will allow you to attach an additional monitor using a VGA cable. Some laptops will allow you to use an S-video adapter and cable to connect your laptop to your TV. Of course you will need to look at both your laptop and your TV to see if you have an S-video connector. Refer to the Wikipedia article for more information http://en.wikipedia.org/wiki/S-Video.
If you have a desktop computer you will need to look to see what kind of connections you have. The connections are typically VGA, DVI, or HDMI. To see what a VGA is go to http://en.wikipedia.org/wiki/Video_Graphics_Array. To see what a DVI is go to http://en.wikipedia.org/wiki/Digital_Visual_Interface. To see what an HDMI is go to http://en.wikipedia.org/wiki/Hdmi. If you have an older computer it will most likely have a VGA connection with is usually blue. Some may also have a DVI connection which is usually white.
In order to use multiple monitors you will need more than one (1) of the above connections. You will for example need both a VGA and a DVI connection. Some of these connections are located in your video card, others are on the motherboard of the computer.
You could always purchase another video card to add more than one connection so you can use multiple monitors. However, you will need to determine if your motherboard has support for PCI, PCI-Express, or AGP and then purchase a video card.
In order to use multiple monitors you must make sure you have two or more monitors both connected to the connections on your computer. You must also have the video driver installed. Many monitors have support for only VGA. You may need a DVI to VGA connector in order to connect a VGA monitor to a DVI connector. However, it is important to mention their are different types of DVI connections. I recommend visiting http://www.datapro.net/techinfo/dvi_info.html to learn about the different DVI connections and determine which connector you have.
It is possible to achieve more than 2 monitors used as once but this usually requires multiple video cards. Typically you can only use two monitors for each video card you have. Having 3 computer monitors used at once may greatly boost your productivity depending on the work you do and time you spend at your desk.