The band

As it usually happens with every band, amplification changes constantly depending on era (i.e. available technology), size of the venue, and of course, how much money are they willing to spend. Queen were, decibel-wise, just as loud as Led Zeppelin, albeit less than Deep Purple. The quality of their sound system live (at least from 1973 onwards) was immaculate, and they definitely had got some very professional assistants and engineers, as opposed to many other bands who may play incredibly well but crew make a total hash.

It's important to note that the way a band works on stage is very different to how they record in the studio, mainly because of the time factor: studio allows experimentation since you can always edit out whatever goes wrong. Concert on the other hand require reliable and comfortable instruments, that's why some may record with triple-H (heavy, horrible and hardly tuneable, yet superb sounding) equipment but on stage prefer something lighter, better looking and allowing more movement.

While live work is more limited regarding certain audio effects, it enables more visuals and audience participation, but for those same reasons equipment radically changes in some cases. In Ogre Battle, for instance, they couldn't reproduce the multi-layered guitars and the crazy sounds of the interlude (which involved loads of screaming and some reversed tom fills); so what they did for that section live was to simplify the arrangement for a guitar/bass/drums power trio, and to use light effects and fireworks to entertain the audience.

Types of instruments

Roger sitting with the instruments the band had during Japanese ANATO tour. From left to right: Brian's John Birch guitar (a Red Special replica), Brian's George Formby ukelele-banjo, one of John's Fender P-Basses, another one of John's Fender P-Basses (fretless type), Brian's Fender Stratocaster and Ovation 12-string guitars, Brian's Gibson Les Paul and a third Fender P-Bass owned by John.

Acoustic: They incorporate their own amplification. Holed guitars, most grand pianos and the vast majority of percussion belong to this category; also most symphonic gear and the human voice. In small venues such as chapels and theaters those instruments may need no further gain than their own natural one. Otherwise the usual way is to use the PA this way: there's a mic near the instrument (or voice) which transports the signal to the monitors and they amplify it through the speakers.

Electric: The sound is produced naturally as well but it's too quiet so it always has to be amplified. Electric guitars and basses belong here, as well as the Gibson Chet Atkins CE which looks like and sounds like an acoustic guitar but it's actually electric (hence CE: Classical Electric). Many acoustic instruments are electrified in order to ease off their amplification: for instance, placing a mic near a guitar hole constrains movement and may cause unwanted feedback, while installing a transducer (like a contact microphone or a piezoelectric pick-up) solves such problems. Same with piano: instead of placing mics at the end of the strings (which caused many problems to both Freddie and Zeppelin's John Paul Jones among others), they'd rather make it work like an electric instrument, placing pick-ups that sensed string vibration.

Electronic: Both sound and amplification is generated by electronics. Those instruments have the advantage that they don't get out of tune (both acoustic and electric obviously do) but on the other hand they can't sound without being plugged. Synthesisers and digital pianos belong to this category, as well as drum pads.

Public Address System (aka PA aka Tannoy)

That's what they use to rock a whole stadium (or some smaller venues)! It consists of a main system (many loud speakers strategically placed for the entire audience to hear properly) and a fold back (sound directed towards performers so they can hear what they're playing or singing and guide each other). Sometimes they send the signal through monitors placed in front of the act, sometimes they use canalphones (which look like hearing aids).

PA Systems used to amplify voices only (not only singing, also for instance in a comference or mass), but from late 70s onwards they control the entire ensemble. For that reason, Brian's guitars were connected to a large set of amplifiers, but as technology got better he merely used one which then sent the signal to the PA and allowed sound engineers to control it with the rest of the gear.

In the really small venues in early days they probably used some sort of simple Tannoy only for the vocals and both basses and guitars would use the amps exclusively. For some 200-300 audiences drums could be loud enough to not require any sort of reinforcement, and that could even be the case with the piano.

Feedback

Because mics (and for a lesser extent guitar and bass pickups) get signal from what they're receiving externally, it's possible that they re-amplify what performers are listening to through speaker/monitors, hence sending it to the Tannoy and creating a loop and loads of noise. While that's sometimes used intentionally especially by guitarists (e.g. Hendrix), it's usually undesirable and disturbing.

For that reason the band never used external mics for acoustic guitars and the concerts where they amplified pianos that way (e.g. Houston 1977) had many noise problems in that department. Vocal mics were relatively few too (usually one for each member and two further ones for Freddie playing piano or guitar).