Uris is revising Bewitched (http://fateordestiny.com/bewitched0.htm), a Harry Potter fan fiction. In this story, Draco is behaving in an uncharacteristic way due to a potion he is unaware of consuming.  She fears that not telling her readers why Draco not acting like himself would chase her potential readers away because they would believe that her character isn't acting as canon and fail to realize that the character is acting that way because he's under chemical influence.  To tell the readers why would be going beyond the character's knowledge and thus violate POV.  We've been asked why is a character doing thus-and-so.  As authors we know the answers, but the characters don't.

We are also writing a Firefly fan fiction, On the Subject of Morals (http://fateordestiny.com/firefly.htm).  Half is written from a telepath's POV.  River can hear everyone's thoughts, but they get filtered through the girl's crippled mind.  So we see how she interprets the other characters' thoughts, not what they are truly thinking.

Let's go over the basic POV's.  We can break these down even more to get odd viewpoints like camera-angle and stream-of-consciousness.  However, the four most used ones are:

First person: I did this.  I did that.

Second person: You did this.  You did that.

Third person limited: He did this.  He did that.

Omniscience: He did this.  She did that while the third planet went around the sun.

The limitations of first person are obvious.  We can only know what one person is thinking unless that character is telepath; the above example, River.  The story must stay in this character's voice.  This POV flows naturally as if we were telling our own story.  Major drawbacks are: it can be too personal and doesn't allow us to know what is happening meanwhile back at the ranch.

Second person POV gives the author a chance to talk to the reader directly.   However, you need to be a literary genius to pull it off well, although we've read books where it is done brilliantly.  Our warning is:  Writing "You are at a feast.  You just ate a poison mushroom and are about to die," may make your readers quite irate.

Third person limited, since you are limiting yourself to a single character, shares some of the same limitations of first person. 

However, since the narrator has his own voice that may be different than the viewpoint character, it is a bit less claustrophobic than first person.  You still can only see what the viewpoint character sees, but you can spread out into his peripheral vision a bit more.

Omniscience is writer as God.  You know all; you see all.  You can see universes.  This gives you the freedom to say:  "He thought.  She thought."  However, this is a hard POV for building suspense because as God you know everything and your readers will feel slighted if you don't have good reason for omitting certain facts.  Warning:  Sloppy third person does not make a story omniscient.

When writing a story, POV is one of the most important decisions a writer can make.  Change the POV; you change the story.  Athena wrote her Smallville/Lois and Clark crossover, Snowball (http://fateordestiny.com/snowball.htm), in two POV's:  First Lois, then Lex.  Although Part 1 and Part 2 have the same dialogue, the stories are very different.  The limitations of POV can be a writer's friend.  A character's lack of knowledge can build suspense.  Through careful use of different POV's, the writer can let the reader in on a secret the character doesn't know and watch in horror as the character steps into a lake filled with alligators.

POV tells you who is telling the story and how they are telling it.  All stories are told by someone.  Stories also can be told by characters within your story.   In the story we're working on, Harry has been questioned by the Minister of Magic once again.  Dumbledore wants to hear Harry's side.

"Harry said, 'A baby was in his mother's arms, and an evil wizard came along.'"  The above example shows that one sentence of omniscience can be more effective than a whole story in that viewpoint.

"And Dumbledore said, 'You don't have to start that early.'"