Basically, diesels' exhaust must be rid of soot and NOx. A ceramic cat can eliminate the soot but the NOx is the hard part. So this technology injects urea (ammonia and water) into the exhaust and a catalyst converts it and the NOx into N and H2O. The website has an excellent BlueTec presentation (aimed at European truck operators) for us diesel voyeurs.
In 1980 when I bought my 300SD, we received a North American directory of diesel fuel stations, I guess in 2007 they'll hand out a similar directory of urea stations - but on a CD for the car's Nav system!