Synthesizers are a huge and complex subject. As a starting point I would recommend this book.
There are many different types of synthesizers and and it is possible for a single synth to provide the sounds of virtually all other instruments. It's simple a question of how much DSP power and memory you want to put into a single machine. In essence a synthesizer is a sound generating oscillator followed by filters (tone controls) and waveform modifiers (attack, sustain and decay). Popular synth models combine multiply oscillators with an internal mixer and effects (reverb, EQ, compressors, etc.) groups. Individual instrument sounds are created by programming multiple oscillators, mods and effects signal pathways and are called patches. Ironically, one of the most common uses of contemporary synths is to emulate the sound of the so-called classic synths.
There are many different types of synthesizers and and it is possible for a single synth to provide the sounds of virtually all other instruments. It's simple a question of how much DSP power and memory you want to put into a single machine. In essence a synthesizer is a sound generating oscillator followed by filters (tone controls) and waveform modifiers (attack, sustain and decay). Popular synth models combine multiply oscillators with an internal mixer and effects (reverb, EQ, compressors, etc.) groups. Individual instrument sounds are created by programming multiple oscillators, mods and effects signal pathways and are called patches. Ironically, one of the most common uses of contemporary synths is to emulate the sound of the so-called classic synths.