Tiff is described as "a typeface diff tool that visually contrasts the differences between two fonts". When you first open the page, the letters R, g, h and e are being displayed in Helvetica ('Font A'). To change the font, click the blue 'Hide' button (which changes to 'Display') and start typing in the field. A drop-down menu of fonts will appear. Make a selection and the letters below will change accordingly.
To compare with another font, do the same in the 'Font B' field and the two fonts will appear next to each other. You can also change the selection of letters at the bottom of the page. And you can see the fonts close up by clicking on the square.
At this point, Google Web Fonts is the only source of external fonts for Tiff, but Yu is working on including more font sources.
Since the app uses some features only available in HTML5 and CSS3, you're best off opening it in the latest version of either Google Chrome or Mozilla Firefox.
