Subtractive synthesis
Subtractive synthesis, the first to be created and the most common one
The subctractive synthesis was the first technology to be used for synthetizers, every instrument mmake a sound made by different base components called harmonics, to be able to recreate sound in a natural way a subctrative synth start using a complex waveform and rich of harmonics (such as a square wave) and using filters it reduce the harmonic content to make it similar to the original sound.
A square wave has every odd harmonic (and they are infinite), that means that in addittion to the fondamental frequency of the sound we can find also a signal of triple frequency and a third of amplitude, five times the original frequency and a fifth of amplitude and so on. This kind of sound is very similar to the sound of pluckes instruments (such as the guitar) and is the base to make sounds with a hard attack since it has armonics with the same phase that are added together. A square wave can be also used to create that typical sound of the computers from the 80s since it was the only kind of signal they could produce (is the simpler one to obtain with digital technology).
Another typical waveform used by subctrative synths is the sawtooth one, this signal differs from the sqare wave because it has all the odd harminics and all the even harmonics. the even harmonics insert some kind of "phase shifts" that create a smooth and full signal.
This kind of wave is often used to reproduce the brass or to create full and big pads, it's also the base for the classic dance lead.
Tje most common filters are the low pass one, used to get natural and full sounds, but very often also other filter types are used such as the band pass and the high pass are used with this synth type. Band pass filters allow us to have a thin and "acid" sound (specially if used with a sawtooth), while highpass give us a cold and light sound.