The origin story of the Hulk is that Bruce Banner is exposed to gamma radiation, which is shown in the opening credits and is not expanded on in the film due to the fact that a Hulk film had already been released in 2003 focusing on the origin. 

In this film, comic book characters introduced are General Thunderbolt Ross (Red Hulk in the comics), Bruce’s love interest and General Ross’s daughter Betty Ross (Red She-Hulk), and the well-received Abomination who is the Hulk’s arch-enemy as well as the film’s villain as well as more inconspicuous incarnations of their comic counterparts, such as Dr. Leonard Samson who has super strength and green hair, Dr. Samuel Sterns who becomes the Leader (hinted at when his head began enlarging due to Banner’s synthesized blood pouring onto it), and Amadeus Cho who becomes the second Hulk who is nothing more than a pizza-eating nerd in this film. 

The Hulk had been established and the big connection to the MCU is shown prior to the end credits- General Ross is sitting at a bar and in comes Tony Stark informing him that a team is being put together. The real reason why Stark approaches Ross is to dissuade him from putting Abomination on the team, which is initially what he was ordered to do by the World Security Council.