Books are generally written from the protagonist's point of view (or POV for short). However, what if you want to switch partway through a book and show someone else's POV?
You can write a book correctly, from any number of points of view, but they have to be packaged neatly. Each chapter or section of a chapter should be written from the same character's POV the whole way through that chapter or section. So if you start writing a chapter from say, a girl named Stephanie's point of view, the whole chapter should only be from HER point of view - what she's feeling, sensing and seeing. The next chapter can then be written from say...Peter's point of view, but then THAT whole chapter has to stay from Peter's point of view. If you want to switch back to what Stephanie or another character is feeling or seeing or sensing, and you want to stay within the same chapter, you either have to cut the chapters in sections with * * * between the paragraphs to let the reader know there's a switch. It's the way to give the reader the heads up that you're changing perspective, so they are expecting the switch, and don't get confused or suffer from what I think of as vertigo. If you're going to do this though, do it consistently throughout the book. This should not be used once, and then not again. Whatever format you want to write in, be in all from one person's POV, from a protagonist and then antagonist, or from multiple characters, just be consistent with the changes and style of those changes throughout the book.
A common and effective way to switch POV in a book is to write complete chapters from different characters' points of view. Robert Jordan does an amazing job of this in the Wheel of Time Series. Each chapter is headed by the name of the person he is writing from, so the reader automatically knows whose head space the story is happening in. Each chapter is told with a different feel, that is consistent with that character. For example, all of the chapters written from Mat's point of view are done in a congenial manner, full of pranks and his perspective on women and gambling and the world in general. Those written from Nynaeve's POV are all no-nonsense, and meshes perfectly with her personality. (If you haven't read his stuff, I would SERIOUSLY recommend it. It's 14 books of awesome, and they're fantasy, and LONG, but his cultures and characters and character lines are so complex (but easy to follow and remember), that his book series has actually been compared to War and Peace. He even goes as far as to take the same readers for all fourteen of the audio books. Michael Kramer reads all of the chapters told from the different male characters' points of view, and Kate Redding reads all of the female ones. It would give you the perfect example of the correct way to write from multiple character's points of view.
The common error occurs when a writer switches in the middle of a paragraph or chapter, with no warning to the reader. For example:
Chad walked down the street with his hands shoved way down in his pockets. The melancholy he was dealing with had stolen his smile, and worry lines creased his forehead. Charlene always walked this way, and he wondered if he would see her. Part of him hoped that he would, just a glimpse of her smile made his heart beat quicker, but her stinging words often cut like a knife; she was not always a nice girl. It usually depended on who she was with. His eyes stayed glued to the cement sidewalk blocks as they slowly moved under him. His long blond bangs hung down, shielding his face from onlookers, and afforded him privacy, allowing him to think. Then, her chiding voice filled the air.
"Anthony, give it back!" she flirted with another boy from their history class. He was holding her book in the air just out of reach, and she was stretching up to take it. She knew full well that she wasn't tall enough to compete with his gorgeous six-foot-two physique, but she didn't care in the least. That wasn't the point anyway, she didn't care about the book, but reaching up like that made her shirt lift, just a little. She saw his eyes flick down to her exposed skin, and caught him smile. She blushed, and took a small step toward him.
"You heard the lady, give it back," another voice ordered from down the block. Ick, it's that annoying creep from history, she thought, perturbed that her fun was to be interrupted...
So you see, I started out from Chad's point of view, and then jumped into Charlene's. For this type of writing to be correct, I should have stayed with Chad's POV and finished the scene. Then I could break it up with * * * or a new chapter and tell a different part of the story from her point of view.(Stephanie Meyers even went so far as to make a start of rewriting the entire Twighlight book but this time from Edward's point of view. I forget what it was called, but it was neat to see the same story from a male perspective).
If you want to see another quick example of how to make the switch in mid-chapter properly, check out the first book in my medieval fantasy series, Knight's Surrender. You can find it in hard copy at Amazon.com (search Knight's Surrender, Heather Reilly), or on ebook at Smashwords.com (Search Knight's Surrender).
Chapters 17, 18, and 19 will give you a great snapshot of how the switching technique is used. Chapter 17 is from the main character's point of view, then chapter 18 switches to the villain's POV. Chapter 19 has the POV switching back and forth, but with those *** breaks I was talking about so the reader sees it coming. I hope you read and enjoy!
No matter whose POV you are writing from, just remember to make sure the whole chapter or scene is complete before you switch. Also, consistency will help the reader know what to expect. When a reader becomes confused when reading, it breaks their suspension of disbelief, and pulls them out of your world and back into the real one. As long as you stick to a pattern, and are consistent with the ways you switch, and whose point of view you are switching to, they will have no problems following your story, and can remain happily submersed in the world you've created.