The program is called Geogebra, which comes from Geometry and Algebra.  Which basically means that it's a visual tool to help you understand the link between these two topics.  For example, telling a student that there are 360 degrees in a circle is one thing.  Having the computer draw a circle and then watching as those degrees appear, counting up from 1 in steps of 45 or 90, brings everything to life.

That is, of course, an extremely simple example of what Geogebra can do, as you can see from the relative complexity of the screen shot below.  The quickest way to get an idea of its features is to search Youtube, where you can see some interesting demo videos. There's a good one athttp://www.youtube.com/watch?v=DUYn3FDo23A for a start.

Here is the GeoGebra home page where you can download a PC version of GeoGebra.  The home page also has additional information on how to use GeoGebra.