tl;dr

Too Long; Didn't Read Used in reply to a post and/or comment or other written material that dragged on way too long about things that weren't very important. Also sometimes used by trolls on posts that are longer than their attention span, or in an argument or flame war when one person can't come up with a comeback. Also usable on your own lengthy post at the end followed by a quick summary of the above material as a courtesy to people who might not want to read the entire thing. Essentially the same as writing "In Summary," when used this way