There is a large twitter community which follows the weekly show and comments on it on #tatort, quite funny and interesting if you speak German. of course the serial itself is on twitter, too - @tatort.
On www.tatort-fundus.de you find more in depth information and Tatort-Fans.de is the site for hard core fans with all useful information about the program and the twitter feeds.
Every episode is self-contained and often takes up newsworthy topics. This might be about practices in the pharmaceutic industries, political announcements or turkish migrants and their problems. The episodes take place in different cities in Germany and their surroundings.
I don't know if Tatort is also known outside the German speaking countries, but I have seen Derrick when I was in Italy. I mean, I have seen that they show it in Italy and apparently it has been quite successful all over the world. A fact that I don't understand because I would have rather bought the Tatort series.
There is also a Tatort podcast which is made very well made.