Photojournalism is a form of journalism that incorporates pictures to tell a story, most often a news story. A photojournalist IS a reporter but he or she needs to be able to make very quick decisions and needs to carry photographic equipment with them in case a photo opportunity arises in relation to their story. A good photojournalist should always report the facts and should never lie to his or her audience. A photojournalist might need to be in situations that are perilous to the journalists life.
While journalism reports the facts through an article, journal, editorial, etc. A photojournalist reports the facts, action, and details as the journalist sees it through a picture. While a photographer takes pictures, those pictures could be of anything, maybe they follow a common theme as in a portfolio but the images have no meaning together. When a photojournalist takes pictures, the pictures must tell a story. They must express the ideas of the journalist through what they can see through the camera.
Hopefully this little blurb has opened your eyes to what exactly photojournalism is and how it differs from regular journalism and regular photography. As you can see, it includes elements of both and is an excellent way to show ideas about current events and stories.