Question

What are voltage sources?

Answer 1

Basically any configuration of electrons that creates an electric field, and therefor a potential difference i.e voltage can be regarded as a "voltage source", but i assume you are speaking in the context of circuits in which case you probably mean something like a battery.

Answer 2

battery cells (cadium, lithium, alkaline, carbon, wet, dry etc.)(chemical)
generators (mechanical rotating)
piezo electric (crystal)
are some.