In many films, TV shows, and books, there exists a character who begins the story as a "normal" person, and then gets caught up in some larger, ongoing conflict. This person becomes the "new guy" on a team of people already familiar with the conflict. This creates a vehicle for the team members to teach the new character, and therefore the viewer, the important points of the story already in progress.
What is the name for this type of character?
The clearest example I can think of is Ellen Page's character in Inception. Sometimes this charcater is also the main character, like Luke Skywalker, Harry Potter, and Neo, but it doesn't have to be.