Queen on stage basically used a primary line-up (drum kit, bass, piano and electric guitar), back-ups (e.g. a spare guitar in case Brian broke a string) and some selective equipment which was employed only for brief parts of the show, or even just for one song (the record was Roger's huge gong, which was only hit twice per concert). Their concerts favoured the visual aspect just as much as the musical, which is why they never tried to replicate the studio versions, and that often included using different instruments to those they had recorded the songs with.
Instruments can be electric (with built-in amplification), acoustic (sound produced naturally but amplified externally) or electronic (both amplification and sound are produced via external equipment), and all of them are in one way or another (via microphones, pick-ups, cables or in recent times midi) plugged in to the Public Address (a.k.a. P/A) which has a main system (loudspeakers strategically placed for the entire audience to hear properly) and a fold back (sound directed towards the performers so they can hear what they're doing, either via monitors or canalphones). Sound engineers play a key role and depending on how they do their job a performance can be enhanced or ruined.
Because microphones (and for a lesser extent guitar and bass pick-ups) get the signal from external stimuli, it often happens that a mic amplifies a sound, then the sound is audible through the speakers or monitors loud enough that it's captured again, and so on, thus creating a loop and a loud noise known as feedback. Sometimes feedback is used intentionally as an effect for guitars (e.g. the intro of Ogre Battle), but mostly they tried to avoid it and that resulted in a lot of their acoustic instruments being amplified electronically, which caused an arguably less natural sound but saved them time, money and effort.