The best and most reliable documents of live shows are the recordings, particularly the unofficial ones, as they have not been altered by the artist for an official release, as has usually been the case over the history of rock music. If it weren't for people taping from the audience and bootleg labels releasing many of the recordings, piece after piece of music history would be lost forever. No artist is ever going to release every recording they've made of themselves, so this is the only alternative for those who want to expand their music collection beyond what has been officially released by their favourite artists.
The overwhelming majority of the sharing and trading of these recordings is done for free by fans who own everything the band has officially released. This simple fact should put to rest those who believe artists are somehow losing money due to the circulation of unofficial recordings. In fact, fans nowadays can watch clips of concerts on YouTube that were filmed with cell phones, and it inspires them to see the shows, which is where artists generally rake in the earnings - not on record sales.
We must begin by dividing the various recordings into two basic groups, and understand why they are different from one another. The two groups are "RoIOs" and "bootlegs." "RoIO" stands for "Recording of Indeterminate (or Illegitimate) Origin." This is a term that originated in Pink Floyd collecting circles when they were trying to distinguish between various types of unofficial recordings. Recordings made from the audience, as well as those recorded from radio and TV broadcasts, count as RoIOs (or VoIOs in the case of videos). These recordings are usually shared and traded amongst fans for free. A bootleg, however, is a RoIO that has been packaged and released by a company that specializes in producing such CDs or DVDs (most commonly in Japan). The http://www.queenonstage.com website is an invaluable resource for keeping up to date with new Queen bootleg releases. I have adopted their colour-coating style for the various major bootleg labels, something which I will not take credit for !
The listings on this website contain both RoIOs and bootlegs. It must be noted, however, that there are still many collectors who create titles and covers for their RoIOs, which often complicate things as they will appear to be bootlegs, especially to people who are new to collecting. Rest assured, only factory-pressed bootlegs are listed here, while all other recordings are given a specific lineage (a piece of information that, when available, indicates exactly where a recording has been between the original source and the listener). See the glossary page and this great (although somewhat dated) analysis for more details on the various terms used in the collecting of recordings.
The quality of a recording from a tape also highly depends on how well the tape was transferred. Here is a great tutorial on the subject:
http://wiki.dimeadozen.org/index.php/Azimuth
Finally, a "Complete" concert may not mean 100% complete. It could mean that 20-30 seconds are missing from the recording (often between songs, when the person making the recording was being conscientious). We must keep in mind that most audience recordings in the 70s and 80s were done on cassette tapes that needed to be flipped every 30 or 45 minutes, so there are usually a few seconds missing here and there. In addition to that, we must remember that people often made much effort to preserve what little tape they had for their recordings (in the 70s, the blank tape or film would cost much more than your ticket!). A few recordings are purposely missing the between-song banter, but most recordings are missing some or all of the cheering between encores. Therefore, if you are a completist who prefers "complete" to mean 100% complete, then there are actually very few "complete" recordings as such. Please keep this in mind if need be.
Click here for the list.