For anyone trying to become familiar with the Civil War concept that is the crux of the upcoming Captain America film, please see the content below as a good introduction of what is going on and where the characters stand on the issue. This was a really great and complex storyline for Marvel comics and there are no doubt major differences to be made for the film. If you want to be completely suprised by the story, then avoid this tread, otherwise enjoy.
Following the events of Age of Ultron, the United States Government moves quickly to implement the Superhuman Registration Act. The Act demands that all Superhumans unmask themselves and register as "Living Weapons of Mass Destruction" as well as revealing their identities to the Government in order for transparency between Superhumans and the people. The Government also wishes to deploy the Act as they see it as a way of ensuring that they are able to monitor when Superhumans aid the American people. However the act also deems those unregistered as rouge superhumans and are considered a threat to the people. This splits the superhero community in half with friends becoming foes. The Civil War is the only known mass war between Superhumans with both sides suffering great loss.
Note* Within the Comic Book Universe the implementation of the SRA is the result of catastrophic event resulting in a huge loss of human casualties.
Anti-Registration (Team Captain America)
The Anti-Registration side believed in the knowledge that their identities be kept hidden from the Government and the people ensuring that their families and friends be protected. Many heroes fled America and sought refuge in Canada but many heroes took up arms against those who were once friends and even lovers. This side was led by Captain America who was originally asked to be the face of the Pro-Registration side but refused believing in the right for himself and other superhumans a chance of a normal life. The Anti-Registration side consisted of many heroes and a fair few number of humans, Coulson, Jane Foster and Darcy to name a few. The Civil War saw many friendships and relationships end. Sue Storm (Invisible Woman) broke off her marriage to Reed Richards (Mr Fantastic) in order to fight for a cause she and her fellow Fantastic Four team members believed in. Pepper Potts surprisingly and despite her relationship with Tony Stark supported Captain America but was not as vocal in her support as other humans. Throughout the events of Civil War many of the Anti-Registration either change sides or bounced between the opposing forces. Within the comic universe the heroes and villains supporting the Anti-Registration cause ultimately surrender after Captain America releasing how much blood has been shed surrenders to the Government.
Steve Rogers (Captain America)
The face of the Anti-Registration movement, Captain America was approached by S.H.I.E.L.D after the implementation of the SRA to be the face of the Act with the Government and S.H.I.E.L.D believing that he would be the most sympathetic to their cause and upon seeing him sign with the Government help convince other superhumans to sign up. However he disagrees, stating that it is his belief that the American Government has no part in micro-managing the affairs of superhumans and people alike and believing the civil liberties upon which the nation of America was founded on. The government quickly moves to incapacitate the Captain who flees and starts an underground movement seeking out friends and foes alike to support Anti-registration. Now on the run from a Government he once believed in, Captain America finds himself fighting against the very people who loved him and considered him a friend. However as always the Captain could have been counted on to rally people to the cause, with superheroes defecting from their own teams to side with him, even those non-superhuman rallied behind the Cap. The Civil War resulted in a showdown of Superhumans and Humans alike with his once close friend, Tony Stark now his number one enemy.
Bucky Barnes (The Winter Soldier)
Bucky Barnes following the events of the Winter Soldier is an interesting addition to Captain America's side, whilst yes he is still Bucky Barnes, he's not the same person Steve once knew. Formerly an operative of HYDRA this ghostly assassin may be one of the very reasons as to why the Government believes so strongly in knowing the identities of every Superhuman. The Winter Soldier in the end credits of Ant-Man has been found by the Captain and Falcon and may with their help regain some idea of who he is but without a doubt is still a massive threat. The concept art despite portraying him as siding with the Anti-Registration, whether or not he will present himself as a threat to Steve or Tony is still unclear however within the Comic Book Universe he does side with the Captain. The Winter Soldier does come under investigation following a major incident within Civil War. So here's hoping to Bucky finally finding himself and choosing the right side.
Sam Wilson (Falcon)
An ex paratrooper, Sam Wilson once again finds himself fighting alongside the Captain as an interesting take upon the comic book version. Sam is one of the few humans in the cinematic universe to side with the Captain, who he sides with and aids in recruiting other superheroes and humans to support the Anti-Registration. He becomes a key member within the Civil War in not only supporting the cause but in being a true friend to Steve. Sam was after the events in the Winter Soldier offered a position of employment with the newly rebuilt S.H.I.E.L.D however chose to decline due to his unease with spy work. Despite appearing in Age of Ultron as a member of the New Avengers Team the events in Civil War may be the breaking point for the New Avengers with its members picking their respective sides. Sam ultimately believes in the Government but not what the Government is choosing to do thus his reasoning to side with the Anti-Registration.
Sharon Carter (Agent 13)
Sharon Carter within the Comic Book Universe is depicted as being grand-daughter of famed SSR Agent Peggy Carter and is a former S.H.I.E.L.D agent who was the love interest to Captain America, within the cinematic universe however she was assigned to watch over Steve Rogers throughout the WInter Soldier. Following the fall of S.H.I.E.L.D she took up arms against HYDRA and supported the Captain and his friends in the retaking and restoration of S.H.I.E.L.D all whilst in the employment of CIA. Her role throughout the Civil War is still largely unknown but due to the fact she's a Carter it's likely she will play a larger role in the film as an awesome kick-ass female character and maybe even as a love interest to Captain America.