Quick Look is a quick preview feature developed by Apple and introduced in Mac OS X v10.5 "Leopard". It is a simple method of previewing documents directly from the Finder.
In Finder simply select an item (document, image, movie, or whatever) and press the spacebar key to view the preview. Quick Look can also be opened in full screen as well as being launched from the command line.
Support for Quick Look depends on having the appropriate plugins usually shipped by the software's vendor. If a plugin is not present you will normally only see and enlarged version of the file icon.