A 24-hour comic is a 24-page comic book drawn in 24 hours. According to the game's inventor Scott McCloud (and yes, this can be called a game, sort of an epic version of Exquisite Corpse or something like it, with the randomness of blind collaboration replaced with the resistance-shattering effects of speed and endurance), you must take your 24 hours consecutively. However, most people who undertake the challenge seem to break the process up into shifts.

As McCloud elaborates at scottmccloud.com:

In the summer of 1990 I was convinced that I was the second-slowest artist in comics. The slowest, my pal Steve "Glacier" Bissette, was having a particularly slow year, producing at a rate of a little over a page a month when he came to the Boston area from his native Vermont to do a signing at a local comics store. I watched in awe as he did sketches for fans. His hands ripped across the page at blinding speed, turning out masterful pen and ink renderings that would make Heinrich Kley weep with envy. I thought: Why is this guy slow?? I'll bet he could do a full length comic in a day if he wanted to! Why, I'll bet he --

(Sound Effect: Lightbulb clicks on.)

Suddenly, I knew what Steve needed to do. And I knew I could only get him to do it, if I did one too.

The deal was struck. We would each do a complete 24-page comic in a day. It had to happen by August 31st. My original idea had been midnight to midnight, but Steve's semi-nocturnal schedule worked better within the more flexible 24-hour rule.

[...]

I finished at about 11:30pm, throwing in a cover so that when Steve did his I could brag that I did one more page than he did. Steve, even though he knew nothing about mine (by mutual agreement), pulled the same trick. Each of us had drawn a complete 25-page, 24-hour comic.

We had no idea what we were starting.

McCloud proceeds to document what he started in the 24 Hour Comic subsection of the Inventions section of his site. Notable cartoonists who've created 24-hour comics include Rick Veitch, Dave Sim, David Lasky, Tom Hart, and Matt Madden. Almost no female cartoonists have taken up the challenge (not and told McCloud about it, anyway), perhaps because the whole thing is kind of an exercise in machismo.

Some of the rules of the game (again from the McCloud site):

No sketches, designs, plot summaries or any other kind of direct preparation can precede the 24 hour period. Indirect preparation such as assembling tools, reference materials, food, music etc. is fine.

Your pages can be any size, any material. Carve 'em in stone; print 'em with rubber stamps; draw 'em on your kitchen walls with a magic marker. Anything.

The 24 hours are continuous. You can take a nap if you like but the clock will continue to tick! If you get to 24 hours and you're not done, either end it there ("the Gaiman Variation") or keep going until you're done ("the Eastman Variation"). I consider both of these the Noble Failure Variants and true 24 hour comics in spirit; but you must sincerely intend to do the 24 pages in 24 hours at the outset.

When you're done, send me a copy.

115 different 24-hour comics are catalogged on the site.