Dōjin music (同人音楽 doujin ongaku), also called otokei dōjin (音系同人) in Japan, is a sub-category of dōjin activity. Dōjin are basically non-official self-published Japanese works which can be based on official products or completely original creations. Such products are sold online on specialized sites, on the author's own sites and in conventions such as the very popular Comic Markets.
Dōjin music isn't a musical genre in itself but is indicative of a particular means of publication. (Similarly the term "indie" in the United States is used in this way.)
Dōjin music consists very often of video game music fan arrangements. Much original Dōjin music also exists, and has been created both for dōjin games and independently, spanning many musical genres such as pop, rock, techno, trance and many more.
By nature, dōjin music is often self-produced, at low cost, by independent artists. Home-studio software is typically advantageous to dōjin music composers, as it is cheaper than studio mastering live instruments. As such, most Dōjin music has a distinct synthetic quality to it. It is common to have one live instrument (such as a guitar) backed up by synthetic orchestrations, though full instrumentation is becoming more and more common in dōjin music, such as orchestral works or dōjin jazz.
Dōjin music artists can be solo or band projects. It is very common for members of different groups to collaborate on an album. Some projects, such as Woodsoft, are collaborations of several artists contributing to a given theme for each of their album releases.
Each member of a group usually have their individual site on which they release their personal works free to download and possibly give updates about their involvement in upcoming albums. Some artists actually never release albums and keep their artistic activity to this free-for-all form. The most productive groups usually release 2 albums a year which are released in summer and winter editions of the Comic Markets conventions and sold for an average of 1000 yen for full length ones. The most involved and popular artists are usually featured on their own group albums but also make guest appearance on other groups' CDs.
Sometimes, people may rewrite the lyrics of an existing anime song to create a dōjin song, or insert lyrics into an originally instrumental anime track. These type of dōjin songs are called "dōjin lyrics" (同人詞 dōjinshi). Many dōjin lyrics are written in Japanese, Chinese, and Korean. Moreover, you can find "fandub" of different languages of an ACG song, Game song or Vocaloid song, like English fandub, French Fandub, Spanish fandub, etc. All of these are Dōjin lyrics of different languages.