NFO files are text-based documents that one usually finds as attachment to other files that contain extended information or instructions regarding those files.
Though you can actually open them with Notepad (they are text files), having a special viewer for NFO files is highly recommended, as they usually include nicely designed ASCII graphics that Notepad doesn't render that well.
Damn NFO Viewer is, as you can tell by the name, a program that has been especially designed to view NFO and DIZ files. It's fast, efficient and simple to use. It's got a similar interface to that of Notepad, and you can tweak custom colors and fonts. It also supports word wrapping.
But that's not all. Damn NFO Viewer also includes some other interesting features, like shell integration (so that NFO files are automatically opened with Damn NFO Viewer) and copy-on-select, which copies any selected text to the clipboard without having to press any extra keys.
In short, Damn NFO Viewer is a small yet very useful tool to view NFO files perfectly.