Snow Leopard Preview: Finder - Icon Previews
One of the handy features that was released in Leopard was Quick Look, which provided a preview technology that displayed more descriptive thumbnails.
Snow Leopard now takes Quick Look to the next level by actually giving a real content preview of the file. For media files, as you move the mouse over the file the “Play” button is displayed, allowing the media to be played directly within Finder. The “Play” button also contains a ring around the button the shows the remaining length of the playing media file.
With document or PDF files, by clicking the floating arrows you can page through the document without opening the associated application.
These new features are another example of integrating tools and functionality into OS X that helps increase productivity and saving the user time by not having to open particular applications when not necessary.