Viewpoint or point of view—shortened in our fast-paced society to the acronym POV—is the angle from which the author writes the story. POV controls the nature of the author's narrative. A change in POV can radically change the character of a story.
Traditionally a scene is written from the viewpoint of the character with the greatest stake in its outcome or whose thoughts and reactions will be of most interest to the reader. It's acceptable, however, to write the first portion of a scene in one viewpoint, and the last portion in another, if events within the scene change what’s at stake.
It’s usually best to write each scene from the viewpoint of a single character. Following this rule allows for greater exploration of character and eliminates the annoying habit known as “head hopping,” or jumping from one character’s viewpoint to another without sufficient reason.