Infodumping is a type of Exposition that is particularly long or wordy. Although it can be done in a way that is unintrusive or entertaining, most infodumps are obvious, intrusive, patronizing, and sometimes downright boring. Specifically, if the premise of your story is laughably ridiculous, an infodump will call attention to the fact. The absolute worst is the gratuitous infodump, which painfully restates that which has already been adequately shown, just to make the reader suffer. For these reasons, 'infodump' is often used as a pejorative.
Intensive infodumping about the world itself (called Expospeak) is most commonly used in Speculative Fiction, where the reader cannot necessarily make assumptions about the way the fictional universe works. Some Speculative Fiction writers, such as Terry Pratchett and Douglas Adams, make a virtue of their infodumping (which can sometimes go on for pages) by making it funny and therefore pleasurable to read. Writers of less comedic works must work hard to avoid heavy infodumping unless they are sure that they can make it entertaining. Many choose to avoid intensive infodumping altogether through the use of supplemental material. If the character does it repeatedly, then he or she is Mr. Exposition.
Specific subtropes that are always Infodumps: