Despite what some people say, head-hopping (shifting POV in a story without appropriate transition) is a fairly common problem in a lot of writing and an issue that every writer needs to pay attention to. Not because I (or anyone else) say so, but because it confuses the reader. I would even argue that a writer who *intends* to confuse the reader ought to be knowledgeable about head-hopping because it could be employed to that end with great effect.
The ever-helpful Jami Gold has posted a concise but comprehensive discussion of POV (Point of View: What Does Your Character Know?) on her blog. The post covers different types of POV (what they look like and how they are used), what can go wrong and suggestions for fixing it, and additional resources. As usual, Jami has done a bang-up job introducing the topic, and if the comments follow their usual pattern, the discussion there will almost be a workshop in itself.