DTM: Deutsche Touring Masters. Production-based (but i think purpose built) german autos. European series
FIA GT Series: It is a new series and only uses GT3 cars
Blancpain GT: Endurance series and different GT-spec cars are used
ADAC GT Masters: Similar to FIA series but sponsore by ADAC auto club.
ALMS: American Le Mans Series. Another Grand Touring series but this one runs 5 classes at once on the track: prototypes and the GT classes. The Rolex Grand-Am series and ALMS will be one and the same next season
WRC: Run by FIA like a lot of these but is a dirt track series. Actually most things except traditional pavement.
Formula 1: In my opinion, truly the gentlemen's series of autosport. This is the highest class with the best drivers in the world. The cars are purpose built for the series by a few manufacturers.
Let's confuse things:
GT/GT2/GT3/GT4- classes based upon modification and engine performance. GT4 is the closest to stock. You are correct about Porsche having a GT-spec car. but so do a lot of other marques. There are GTO, GTB (B = Berlinetta hence the F12 Berlinetta being the newest GT model from Ferrari), GTC, GTR and of course BMW's M3 GTS.
For GT4 look at this description:
http://www.bmw-motorsport.com/en/cars/bmw-m3-gt4.html
Pro-Am: When an amateur shares the cockpit with a professional driver. Blancpain, for example is Pro-Am. Usually they have to stop and switch drivers at some point. It's series like this that enable show hosts to run a race for a show segment.
Alright, I'm not good at explaining things so that's the best I can do for you lol