Final answer:
The protagonist is the main character of a story, which could be an individual or a group, and doesn't necessarily have to be the 'good guy', as exemplified by Satan in 'Paradise Lost'. An antagonist opposes the protagonist and could be a person, group, or concept. Secondary characters also play significant roles in the plot.
Step-by-step explanation:
The protagonist is the main character(s) in a story, around whom the plot revolves. The protagonist is often but not necessarily the 'good guy' and could be an individual or a group. An example to illustrate this is John Milton's 'Paradise Lost', where Satan is the protagonist even though he is not traditionally viewed as a hero. It is important to understand that a protagonist is the focal point of the conflict in the narrative. Furthermore, there could be multiple protagonists if a story contains a series of conflicts.
The antagonist is not necessarily a person but could be a group, a nation, or even a set of ideas that stands in opposition to the protagonist. While the protagonist and antagonist are crucial to the development of the story, other characters such as secondary or minor characters play supporting roles or may not directly interact with the main characters but still contribute to the plot progression.