Thanos, the purple smiling guy in the stinger after The Avengers, will appear this summer in Guardians of the Galaxy. The big bad of the entire Marvel Cinematic Universe will be voiced by Josh Brolin (Oldboy 2013). Thanos was portrayed by Damion Poitier during his brief cameo in Avengers.
So far in the MCU, Thanos is responsible for giving Loki his staff and his Chitauri army. In the comics, Thanos has a history with the Guardians of the Galaxy as well, he killed Drax’s family and adopted Gamora as his daughter to train her as an assassin. Thanos’ plans often involve uniting the Infinity Stones with the Infinity Gauntlet, all of which have been referenced by the Thor movies. We know that the Gauntlet and the Cosmic Cube/Tesseract are in Asgard while the Aether has been given to The Collector (Benicio del Toro) for safekeeping. Marvel has also confirmed that the orb Peter ‘Starlord’ Quill attempts to retrieve in the Guardians trailer is another stone. Finally, some have speculated that Loki’s staff contains yet another stone.
It remains to be seen how much of Thanos’ backstory will get brought up in the movies though it seems we may not have to wait until the next Avengers movie to find out. Guardians of the Galaxy will arrive in theaters on August 1 followed by Avengers: Age of Ultron on May 1, 2015.