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, and there may just be situations where it's a Necessary Weasel due to there being no other way to more naturally explain the plot point over the course of the story before it becomes critically important without bloating/slowing down the pace of the story, 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. Even worse, one character may be saying it to another who is fully aware of it already, for no good reason besides filling in the audience. Having one character be The Watson is a common workaround to this, as their presence creates the need for other characters to exposit.
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.
In a video game, infodumps are normally in the form of a long, boring lecture from one of the characters babbling on and on, giving you pages and pages of trivial information that may or may not be useful to the player. May include Shall I Repeat That?
Specific Sub-Tropes that are always Infodumps: