Agnivarsh, a scan converter is a device that ususally takes a VGA-input (from your video card), and converts the signal to an S-Video/RCA output for your TV; in other words, it allows any video card's display to be shown on a TV screen. If you know what a JAM! board is, this does the exact opposite!
By the way, most scan converters are mediocre compared to good TV-out cards (the quality of the conversion is on-par with a TV-out display from a diamond V550 or V330), but some more expensive ones work really great; plus you get a lot more features than a simple video card with TV-out, such as simultaneous display on both monitors and TV (with any video card), manual zoom of one part of the display onto the TV screen, readjustment of the refresh rate for sharper/smoother picture on the TV, etc... They work with games or any programs, but their original purpose was for allowing people to present their powerpoint slides onto a big screen TV (just because projectors are so damn expensive).