The first season of Attack on Titan aired from April 6, 2013 to September 28, 2013, as stated on Wikipedia. Although the anime is popular and there are demands for the second season, the second season is not yet released. I came across this article, which explains the reason for the delay as:
According to the director, there are a lot of things happening in the "Attack on Titan" franchise this year, which hinders the second season production and has delayed the hit-anime to 2016. It was also revealed the delay is due to Hajime Isayama's best-selling manga lacking two-story arc, which make it impossible for Araki to come up with a new season.
This makes fans (like me of course) impatient... What is this lacking two-story arc referring to?