For the outsiders, the words 'Comiket' and 'dojinshi' probably do not ring any bell, but in Japan it has become a huge phenomenon.
 
What is a 'dojinshi'?
It is the Japanese word for 'fanzine'; 'dojin' literally means 'peer', someone who is the same as you, a comrade, and also indicates a literary group, while 'shi' means 'magazine'. Manga have a heavy influence on Japanese culture, though not always obvious, and even the few people who never read a manga are aware of them and know exactly what 'Jump' or 'Dragonball' are. (Namely, the n.1 selling weekly manga magazine and the world's all-time manga and anime bestseller from the 90s). Many people are not satisfied by only reading them, though, they also want to draw their own stories, using their favorite characters from existing commercial works, or developing their original ones, creating something completely new.
There are also text-only dojinshi: short novels, short essays, poems, often enriched by some black and white illustration, or a mix of both manga and written text pieces. You can find fan-fiction as well as original fiction.
 

In a country like Japan, where copyright is almost a deity in its own right, as foreign publishers have learned very painfully during the years, undergoing long and meticulous approval processes every time they wanted to do anything with a Japanese property, it seems impossible that Japanese publishers would allow amateurs to use their beloved, money-making characters as they please, infringing all sorts of copyrights… but hey, this is another one of those charming contradictions that make this country so interesting. In truth, the publishers close an eye or two over the dojinshi phenomenon, because more than anything it is good promotion for their titles; especially nowadays, with the global financial crisis and a steep drop in magazine sales, dojinshi help circulating information and keep the interest high in titles that probably would have a harder time getting, and especially maintaining, exposure without that kind of underground 'word of mouth'-like promotion. Actually, 'undergound' doesn't sound too right for a gigantic phenomenon reaching hundreds of thousands of people.
 

But what is *really* a dojinshi? What is it all about? What does it look like?
Dojinshi nowadays come in all sorts of shapes and glitzy material, mainly in B5 format counting 20-60 pages each; the covers are generally in color but the manga are mostly in black and white, in the Japanese tradition. The circles who produce them could be groups or individuals, and the first publish also several authors' works collected together in what they call 'anthology' books. These are usually themed anthologies, for instance 'megane' (guys wearing glasses), 'nekomimi' (cat ears), etc. Anthologies are generally bulkier than 'simple' dojinshi, and the best-selling circles do reprint out-of-print series collecting them in bigger volumes, sometimes changing the format. The contents can be anything, from action to romance, from totally innocent to sexually explicit to downright perverted (the cover duly states "18 year old up"). Dojinshi are influenced by commercial anime and manga, but they also influence them back, and many cultural 'booms' originate here: moe, lolicon, nekomimi, megane and other major and minor trends which flow out in the mainstream.

Basically, there is no rule, as it is a free market created by amateurs, it doesn't have to bend to the rigid rules of business. Though in the end, it is a business. The numbers speak clearly.
 

What is Comiket?
http://www.comiket.co.jp/index_e.html
It is short for 'Comic Market', the Mecca of the dojinshi fan since 1975. In the first six years, there were 32 circles for a total of about 500 people, with an attendance of 7000 to 10,000. Quite good figures for an underground phenomenon in such analog age. It was the time of the Uchuu Senkan Yamato and Gundam booms, and living legends like Moto Hagio, Keiko Takemiya and others were revolutionizing the Shojo Manga giving birth to something that would change the history of Japanese manga for women forever. (See: Yaoi!!). At the beginning, the circles were membership-based to offset the very high printing costs, original works prevailed and parodies were not yet so popular. Gradually, with the expansion of the Comic Market thanks also to the bubble of the eighties, the groups started getting smaller and smaller, and single author circles emerged, as dojinshi became a real business, and the bestselling names could make a living just by drawing and selling their amateur work.
 

Nowadays, a lot of famous professional authors come from dojinshi, and actually many of them still produce them, juggling within the hard schedule of the real mangaka. "Why?" - would you ask… if you go to the Comiket, maybe you'll understand. It is not just the writing, drawing, making and selling your own thing that makes the dojinshi world, but the getting together, all with different tastes and styles, but with one big passion, and the happiness to share it with alike-minded people. Now Comiket counts two yearly editions, 35,000 circles (many more apply every year), 560,000 general attendees, but the spirit, in the end, remains the same. This is probably the most amazing thing about this amazing phenomenon.
 
Comiket is held every year at Tokyo Big Sight in the bay area; Natsucomi (Summer Comic Market) around O-bon (August 15th) and Fuyucomi (Winter Comics Market) around New Year's. Natsucomi sees the most foreign visitors, so if you want to dive in the international experience, brace to fight the heat and enjoy outdoor cosplay!
http://www.bigsight.jp/english/general/access/index.html
 

Cosplay?
It is a sin not to mention cosplay when writing about Comiket; they basically grew 'up' together. Short for 'costume play', originally means wearing a costume (for halloween, at a carnival, dress up as a doctor, whatever), now it is mostly used to indicate dressing up as a fictional character; would that be from a manga, animation, videogame, even feature film, etc. Generally, cosplayers go to the event in normal clothing, carrying a bag or a trolley and they change there, then they gather in a designated space where they can pose, alone or together with others, and have their picture taken by other attendees.

It's quite fun to go around spotting strangers who are dressed as characters from your same anime, taking pictures together and making friends. It is generally strictly prohibited to exit the premises in cosplay: everybody has to change back before leaving. Apparently, it is because the organizers do not want the 'normal people' to think that cosplayers are crazy perverts. Many people go to cosplay events only to take pictures, and many do it for the fun of seeing semi-naked girls posing as their favorite manga hottie or even just to take pictures of whoever is cute and semi-naked. Thankfully, not everybody is so narrow-minded, and many people go to cosplay events for the same reasons why they go to Comiket: to share their passion with like-minded people.
 

Traditionally, cosplay has been seen in dojinshi-related events, but nowadays it has got so popular that cosplay-only events are held regularly around the country. And also around the world. Since 2005, the city of Nagoya hosts the World Cosplay Summit, and awards the world's best two-people cosplayer team, among candidates from all parts of the world. The competition is actually quite fierce, and each country holds selections months ahead to pick the lucky pair that will get a free trip to Japan to proudly represent their nation. Comiket itself is getting pretty international, enjoying more and more foreign visitors and also foreign cosplayers every year. Some of them come for the cosplay, some for the experience, some to buy dojinshi, but the latter are not necessarily all able to read Japanese. Art knows no boundaries!