The Avengers is a group of rebels that has been made up of the old Avengers and mutants. Once the Skrulls invaded and made their intentions clear, the time for division due to differing opinions on the SRA quickly came to an end. They needed to make peace with one another and started to work together to free their planet of the green menaces.
The Unity Division is a newly formed team that is based on the cooperation of mutants and the Avengers. Because of this, they are a unique sub-team with their own leader in the form of Magneto. They are primarily made up of mutants but the team is open to membership of anyone that would qualify to join. They are based out of Xavier's School for Gifted Youngsters.
Mutants are a sub-species of the human race. These are people who have an activated X-gene, which gives them their mutations. These mutations can range from purely physical to giving the person amazing abilities. For some time the mutant race has lived in the shadows and hasn't made themselves apparent to the general population. With the Skrulls invading and wanting to use humans as a resource, mutants will likely be hunted for their uses as potential weapons.
These are people that have gained their powers through use of science, technology, skill or magic. They have extraordinary abilities that allow them to do amazing things. While most try to use their powers for the better, there are others that wish to use their abilities for their own gain. This is part of the reason that the SRA was passed. However, times have changed and the invading Skrulls has forced those with superhuman power to band together.
Civilians are people without powers. They generally are the ones that are being saved or harmed during incidents involving Superhumans. While they were a major proponent for the SRA being passed, their opinions regarding Superhumans has almost been forced to change due to the invading aliens.
An amalgamation of anyone that is affiliated with Apocalypse's regime. That includes the immortal pharaoh himself, his Horsemen, and anyone that might join the army. Apocalypse will not accept normal humans among his army unless they have proven themselves to him.