How is sound actually recorded?

Question

I know that sound was first recorded by Edison, but how did he do it?

Answer 1

Two techniques: analog and digital.
Analog: the sound is a variation of air pressure. This variation is "measured" by a microphone (or similar), to convert the pressure differences into variations of voltage.
This voltage variation is then used, after amplification, to act upon a "stylus" (in the case of Edison) that "vibrate" on a cylinder of soft wax, animated by a rotation and a horizontal translation. The variations of the stylus can be read afterwards by retarting the process, this time fromt the stylus to a speaker.
Later, the voltage variations where converted in magnetic variations that were "written" to a wire first, then to a tape (tape recorders).
Nowadays, the same voltage variations are "sliced" and measured many times per second, converted in numeric value, and it is the number that is recorded on some disc or solid state memory: this is the digital recording.