The Comic Market (Comiket) is a marketplace where individuals can offer their own self-produced creations to a community that appreciates and supports such creative personal activities. Comiket primarily focuses on acting as a communal exchange place for self-published books, known as dojinshi. Japanese dictionaries define dojinshi as self-published material aimed toward like-minded individuals and the Comic Market invites any publication (books, music, films, software, etc.) published by individuals that are not available through standard commercial outlets. Applications for booths are processed prior to the event. You cannot apply for a booth at Comiket itself.
Source: official website