WEC (FIA World Endurance Championship) - being run for the first time in 2012. Developed from the ILMC (Intercontinental Le Mans Cup), which was successfully run in 2010 and 2011. An official FIA series in 2012, but run in collaboration with the ACO. Website: www.lemans.org (official website is www.fiawec.com, but this is still "under construction). Eight races planned for 2012, mainly in 6-hour format, except for 12 Hours of Sebring (shared with ALMS) and 24 Hours of Le mans. Controversially, Petit Le Mans will not be shared with ALMS this year. Classes: LMP1 (incl. the factory teams, e.g. Audi and Totyota, but sadly not Peugeot in 2012; Audi and Toyota entering hybrids) / LMP2 (privateer teams, running chassis from Oreca, Lola, Zytek, HPD, etc) / LM GTE Pro / LM GTE Am. The E in GTE stands for Endurance and these classes have taken over from the old GT2, which is now defunct. GTE Am teams have to include at least one amateur (gentleman) driver. Porsche will compete with the GT3 RSR, with the Felebrmayr Proton team being the top Porsche team.
ELMS (European Le Mans Series) - used to be known as the LMS (Le Mans Series) until 2011. Run under the auspices of the ACO. Five 6-hour races planned for 2012, all in Europe. Website: www.lemans-series.com. Classes: LMP2 (private teams only) / LMPC / LM GTE Pro / LM GTE Am / LM GTC. LMPC and GTC are the "Challenge" classes and are meant to provide a cheaper entry into the series (LMPC runs the common-platform FLM (Formula Le Mans) cars. No LMP1 in 2012. Again, Porsche will be running the GT3 RSR in GTE Pro and Am classes, with Prospeed Competition being the primary Porsche team.
ALMS (American Le Mans Series). Similar technical and sporting regulations to those applied in WEC and ELMS. Ten races took place in 2011, starting with Sebring (12 hours) and ending with Petit Le Mans (1,000 miles). Website: www.americanlemans.com. Classes: LMP1 / LMP2 / LMPC / GT / GTC. GTC (GT Challenge) is again an entry-level class, and competitors raced the Porsche 911 GT3 Cup in 2011. Porsche will enter the GT3 RSR, with the Flying Lizards being the top Porsche team.
FIA GT1 World Championship - 10 races planned for 2012, including India, Argentina, etc. Format is 1-hour races, with a qualifying race on Saturday and a championship race on Sunday. Website: www.gt1world.com. Big change in 2012, as the series' sporting and technical regulations are being unified with the GT3 European series - so, although called the GT1 championship, the cars will effectively be GT3 spec. Porsche has not yet confirmed its participation, but if it does, it will most likely compete with the GT3 R.
FIA GT3 European Championship - 6 races planned for 2012, all in Europe. Same format as GT1 above. Website: www.gt3europe.com. Porsche will run the GT3 R in this series.
Blancpain Endurance Series - only run since 2011. 6 races planned for 2012, following a 3-hour format, except for the 24 Hours of Spa (the showcase event of the series). Organised in parallel with the GT3 European Championship and the GT4 European Cup - so classes are GT3 and GT4. Website: www.blancpain-endurance-series.com.
In addtion to the above major series, there are a host of other GT and endurance series which are worth following, inlcuding the GRAND-AM Rolex Sports Car Series, the International GT OPen, the ADAC GT Masters, British GT Championship, Japan Super GT, the VLN series at the Nurburgring, and of course the legendary Nurburgring 24h.
Hope this helps. Can be quite confusing, especially given the changes coming in 2012 (and I'm sure I got some things wrong). We're still waiting for confirmation of events and entrants in some of these series. 
