Literature Tropes #2: Mr. Exposition
Mr. Exposition. This trope involves a character who exists only to explain a plot element, an important scientific or magical law, an aspect of a foreign culture, etc. to the protagonist. Often it's actually for the benefit of the reader, not the protagonist, who should know it already. Think of all those TV detectives explaining forensic evidence to each other. I call this "exposition in dialogue," and I hate it if all characters in the conversation already know everything. As a writer, you should think of another way to get the information across to your reader. If it's ingrained enough in the character's personality, however, it can work. Think of Star Trek's Data or the immortal Sherlock Holmes.
Another variation is Captain Obvious , a character who says something that the reader and all present characters should clearly know. Picture two people tied to the tracks and blinded by the light of the oncoming train. One character says, "Hey, there's an oncoming train!" This can be done for comic effect, but avoid it if it's not funny.
For more, see tvtropes.org.