xkcd is a webcomic drawn by Randall Munroe, hosted at xkcd.com. It focuses on science, mathematics, computers, and general geekiness, told with a light, quirky sense of humor, and at times profound philosophizing. Its art style is minimalist, told through simple stick figures. New comics are posted every Monday, Wednesday, and Friday, with each comic accompanied with title text serving as Randall's commentary.
xkcd began as a series of Randall Munroe's hand-drawn sketches which were first posted on Randall's LiveJournal account named xkcd_drawings. The first sketches were posted Friday, September 30, 2005. Randall posted 13 sketches that night from 8:34 pm to 8:46 pm. Most comics were titled and accompanied by a comment from Randall (similar to what would become title text in xkcd). The 8:40 pm post included the first link to xkcd.com in respect of a comic strip drawn during a NASA lecture.
The next post was on Sunday, October 2, 2005 in which Randall indicated that he would begin posting comics on Monday, Wednesday and Friday (the post, made just before midnight, was Monday's drawing). xkcd has generally been posted on a Monday-Wednesday-Friday schedule ever since, other than during the 2005 winter break between December 5, 2005 and January 4, 2006 when no comics were posted.
Randall posted comics on LiveJournal until Monday, January 30, 2006, when Randall announced that he would be posting his comics on xkcd.com and had set up an RSS feed and an automated posting method with the help of someone named Derek. Randall created a syndicated RSS feed account for LiveJournal called "xkcd_rss". The original LiveJournal account was therefore no longer necessary and would be shut down. The next post, on Sunday, April 23, 2006, advised that Randall was about to delete the original LiveJournal account, and credited "davean" for helping with the automated posting script. For unknown reasons, all of the comics originally posted before January 1, 2006 (i.e. before 45: Schrodinger) were all dated January 1, 2006 when the comics were posted on xkcd.com, and the strip numbering of those comics are not all in the same order as originally posted on the LiveJournal account.
When comics were archived at xkcd.com, they were given numbers which did not entirely match the order in which the strips were originally posted to LiveJournal. As it is currently numbered, xkcd premieres with Barrel - Part 1 - the first part of a five-part story whose parts were distributed amongst the first thirty-one strips. Following the premiere strip, five of the next seven strips are simply one-panel sketches with no hidden meaning or comic purpose. One is clearly marked as a sketch from Randall's 11th grade Spanish class which was originally the first sketch posted on the LiveJournal account. This comic clearly was a scanned pencil sketch with slightly-askew graph-paper grid visible.
The light-blue graph paper grid were a part of the style of early xkcd, beginning with the 2: Petit Trees (sketch) through 22: Barrel - Part 3 (except for 15: Just Alerting You). The grid evolved to the point where in some strips in only appeared without the outlines of the individual panels of a strip. It is unclear if all of the strips were drawn on graph paper and the grid was scanned in, or whether some had the grid added digitally.
For the next twenty strips, there was inconsistent use of the grid; some strips had no grid; others had a black grid; others had the normal light-blue grid; on some the grid was faint as if it was poorly scanned or Randall had attempted to erase it (he noted having done this on 11: Barrel - Part 2). The last comic (by number) to use the grid was 44: Love, which was actually posted relatively early on the LiveJournal account but received a high number when the comics were renumbered. The last comic by date to use gridlines was 39: Bowl, although even these comics may have been posted on LiveJournal out of order. The faint remains of gridlines in comics 26: Fourier and 27: Meat Cereals suggest that Randall may have erased gridlines in these and other comics as well. In 2012, Randall revived the blue grid as a background image for what if?.
Similarly, the art process has evolved. The first comics appear to have been physical sketches (pencil or ink) which were scanned and directly posted; although some comics occasionally featured a digitally added caption. The first comic that appears to have had digital inking and effects (although it could also just be a different physical medium) is 23: T-shirts. The comics continued to be hand drawn, and hand lettered, with possibly a bit more digital polish. The comic slowly evolved to apparently become entirely digitally inked and lettered. Although early comics included sentence-case text, beginning with comic 90: Jacket, Randall began using all-caps (although it took several strips for the all-caps to become the standard format).
xkcd comics are usually plain, black-and-white line drawings, featuring stick figures, some of whom form a cast of recurring characters. From time to time, Randall posts things that aren't his every-day comics, but does something a little above-and-beyond. Whether it's a case of user-interactivity, leveraging certain behaviors of HTTP or the graphic medium, or even just commemorating the passing of an influential person, these few comics stand apart from the usual Cueball and Megan antics.
 Meaning of "xkcd"
According to the xkcd FAQ, the name "xkcd" is "not actually an acronym. It's just a word with no phonetic pronunciation -- a treasured and carefully-guarded point in the space of four-character strings."
However, it has been noted () that if each letter of the alphabet is mapped to 1 through 26, the sum of the values for X, K, C, and D is equal to 42. The value may be significant since this is the answer given to the Ultimate Question of Life, The Universe, and Everything by the supercomputer in The Hitchhiker's Guide to the Galaxy. In April 2005 the film adaptation opened, the same year in which Randall later posted his first comics to LiveJournal. The book is also referenced in 548: Kindle.
The FAQ also mention that the preferred form to write "xkcd" is in all-lowercase, except in the beginning of sentences, where it may be styled "XKCD" if required and "Xkcd" should not be used. However, Randall himself sometimes breaks this rule in the comic transcripts, as can be seen on this list. In the comics themselves, since all text is written in all caps, "xkcd" naturally also assumes that form, without, however, violating the prescribed style.
See also 207: What xkcd Means.