xkcd is a Stick Figure Comic by Randall Munroe. It is a gag-a-day comic, and generally does not have a continuing plotline or continuity (though there are occasional short story arcs). Many of the jokes are based on math, physics, UNIX and Internet memes, as well as romance and sex.

Originally a relatively unknown set of personal sketches and doodles, it grew in popularity in 2006 when other webcomics (such as Dinosaur Comics) began linking to it. However, it was when Randall posted a "Map of the Internet"*, and said map was subsequently featured on Slashdot**, that xkcd's popularity truly erupted. Since then, it has been among the most well-known of webcomics.

Of course, you wouldn't know that just by looking at the comic. The characters are still drawn as very basic stick figures, with no facial features other than hairstyle (which is often used to distinguish males and females). However, there are three recurring characters who can be recognized by their respective headgear:

• "Black Hat Guy," a Jerkass Badass character with a black pork-pie hat, who in one recent storyline encountered a woman who out-Jerkassed him, and has now become a romantic interest.
• A beret-clad Cloud Cuckoo Lander, generally thought of to be an Existentialist, albeit one with a thing for pastries.
• A dark haired woman, referred to in several comics as "Megan" she shares many of the same interests with the nondescript Author Avatar and is commonly shown to be in a relationship with him. Was the main character of the "Choices" Series.

There are other recurring characters in the same social circle — e.g. the dark-haired existential nihilist — but most of them are less distinctive.

xkcd is part of the documentation for goto on the PHP website, and was mentioned as a ticket in a changelog.