Its the standard Great Stuff brand expanding urethane foam. Its water proof, easy to use and very cool to work with. I actually got the idea from this very site. I usually use real rock and the like, but weight quickly becomes an issue.
Here is the thread that I gleaned ideas from:
http://www.caudata.org/forum/showthread.php?t=38607
There is a few more, but I recommend trolling through that section of the forums, there is a mountain of really great techniques and ideas there.
It is water proof all on its own, no sealer required. However there is quite a bit of trial and error to using it, as it expands rather randomly. There is also a bit of shrinkage once it is in water too, though I think this was due to a design flaw I made due to water weight and plant/soil weight.
This was the first time I used Great Stuff like this. My next project, I am going to create a basic framework of polystyrene foam sheeting first and apply the foam over top of that. Cracks formed in the surface which dried solid before the core of the thick parts did. I had to reseal those cracks with aquarium sealant to keep the water from draining into the voids left for planting.
Believe it or not, this really was a simple project- the painting technique makes it look like it takes for ever. 2 hours of prep and work to get the filter and foam in place. I let it dry overnight, then painted it (about 20 minutes of painting). I let the paint dry for 12 hours, then sealed it with a clear urethane spay coat and let that dry.
Practice with the foam a bit before you start on the tank. With any brand of spran foam, once you open a can, you have to use the whole thing or it seals itself shut and wastes the unused portion.