"TL;DR is an observation very much like the complaint that Mozart's music has too many notes."*
Originally the comment "Too Long, Didn't Read" or "TL;DR" was a snippy way of saying that you hadn't read what the previous poster had written. In theory this was because they took too long to get to the point, but in practice it was usually because you didn't care what they had to say. In such a case it is obviously a comment only used by trolls. However, it was recognized that in some cases the trolls had a point; people are less likely to read essays than quips, and therefor a poster might well want to shorten their comments. However, very few issues can be completely expressed in a sufficiently pithy way to please a generation raised in front of the television, so a new procedure emerged.
It is now common on many sites (Not E2!) to write your piece and, if finding it unduly lengthy, to add a short paragraph to the end, with the heading tl;dr and containing a brief summary of the content. It is becoming more and more common to actually include this summary at the beginning of the piece, a practice of which I approve, as it helps orient the reader and saves the lazy reader from unnecessary scrolling.
The original use of tl;dr is said to be found on the General Mayhem website, posted by user waptang on June 19, 2003. It spread quickly to sites like 4chan, SomethingAwful and FARK, and has since appeared on just about every site that has a discussion forum. As a general rule, tl;dr is always rude unless followed by a summary of whatever text was tl. It may be considered rude if posted by anyone but the author. On the other hand, it is considered good manners for the author of a long post to tl;dr his own monologue when appropriate.
Variations include TLDR, TL/DR, TLDNR, TL;DC, TL;DW, TL;DR;LC, and TL;RA along with the lower-case versions of all of these. Because internet users are very strange and rather clever people, sometimes a macro of a teal deer is used instead (read tl;dr out loud).
TL;DR This is used either to insult the author of a too-wordy post OR as a way to summarize your own long post.