An argument is a collection of one or more viewpoints organized into an outline. The first viewpoint represents the writer's point, and the supporting viewpoints represent the reasons why the argument's point is valid.
Arguments are what Goodpoint was designed to host. They are Goodpoint's "posts." A single viewpoint is also an argument where the point is being made without any additional, supporting viewpoints.
A rule of thumb for ensuring that your point is an argument: your argument is something that a reader should be able to agree or disagree with.