Main characters are central to the plot of the book. They appear frequently, are described in detail, and figure prominently in the story line. Minor characters, in comparison, do not appear frequently, are usually described in less detail, and have limited roles in the story line.
A single character who is most prominent in the plot may be referred to as the 'protagonist', the 'main character' or the 'focus character. An example of this is the character of Scarlett O'Hara in Margaret Mitchell's "Gone With the Wind" or Mowgli in Rudyard Kipling's "The Jungle Book".
Round vs. Flat
The distinction between round and flat characters was introduced by English writer E. M. Forster. He introduced the concept of round characters as referring to whose characters who have complete, fully-rounded personalities displaying complexity and depth. Flat characters, in contrast, are simple and one-dimensional. They tend to be seen in terms of the single side of their personality which they present to a more major character, or which motivates actions that affect the plot.
Round characterizations are necessary to capture the reader's interest in the major characters of the book. These characters' motivations, or changes in their thinking, typically play critical roles in the development of the story, and are thus laid out in detail. Think of the example of Scarlett O'Hara or Melanie Wilkes in Margaret Mitchell's "Gone With the Wind". Flat characterizations are normally saved for minor characters who play specific roles relative to the plot, or have specific (usually simple) relationships with the main characters. Returning to the example of "Gone With the Wind", flat characters include both friends (Prissy, her many suitors) and enemies (India Wilkes, the overseer's wife) of Scarlett O'Hara.
Sympathetic vs. Unsympathetic
True to their names, sympathethic characters are written to encourage the reader's sympathy and support while unsympathetic characters are intended to raise the reader's ire. In both cases, they serve to increase the reader's level of emotional involvement in the book and the degree of "realness" which he or she experiences, as well as serving to illustrate the author's theme(s).
The importance of sympathetic characters is therefore two-fold. Most obviously, sympathetic characters can represent the moral (presumably winning) side in the battle between good and evil. In short, sympathetic characters are the story's "good guys", "cowboys in white hats" or "knights in shining armor".
But even more importantly (and true to their name), sympathetic characters attract the reader's sympathy and, by doing so, draw the reader into the story. By providing a character whose feelings and dilemmas the reader can relate to -- or even aspire to -- the writer succeeds in keeping hold over the reader's interest and attention. The protagonist and namesake of Henry Fielding's "The Adventures of Tom Jones" is a case in point: although Jones' character has many flaws as a person, our sympathies are such that we want things to come out well for him in the end.
Types, Archetypes and Foils
These terms all refer to characters whose personality is secondary to their purpose. In short, they are categories of characters who are not necessarily important in and of themselves, but rather are used as tools to make specific points relative to the book's theme, to help set mood, to encourage the plot, or to draw out the details of other characters.
The Type is modeled after one of any number of well-known categories of character that have evolved from our long history of literature. Examples include "the hooker with a heart of gold", "the jealous stepmother", "the rebel without a cause", etc. Such characters are often portrayed in a flat manner and run the risk of being clichΘs, since their behavior is predicated upon commonly-held expectations.
The Archetype goes one step further, referring to a literary creation that is more of a symbol than a credible character. Archetypes are typically a reflection of our own social or psychological needs, and are used to influence both the reader and the other characters. Examples include the Virgin Mother, the Devil, the Immortal Child, etc.
The Foil's main purpose is to highlight or draw out the features of another character. This is typically done through a contrast of personalities or a clash of motives that places the foil in direct conflict with a main character and, by doing so, reflects back the strengths and weaknesses of the more dominant (and important) characterization. Foils can be used to illustrate dichotomies such as good vs. bad, strength vs. weakness, intelligence vs. stupidity, worldliness vs. ignorance, etc.
