Try putting a finger on your Adam's apple (your vocal chords are inside it) and saying a long deep ooooh. Then try whispering it.
See? No vibration if you whisper.
But of course it's not as simple as that. In the English sound b, for instance, the vocal chords start vibrating part of the way through saying it. The sound p isn't voiced (ie the vocal chords don't start vibrating) until after it's finished.
An s in English may be voiced (bugs) or not (butts).
But English is simplicity itself compared with some other languages. The Mexican language Mazatec uses, as well as our voiceless and voiced phonation, breathy, slack, stiff, and creaky ones. In the Bor dialect of Dinka, spoken in South Sudan, whether you say a word in an ordinary-voiced, breathy, harsh, or yawning phonation might make it mean diarrhoea, go ahead, scorpions or to swallow - which could, obviously, be a matter of life or death: or, possibly even worse, really serious embarrassment.
It's all rather wonderful, isn't it?
Thing To Consider Today: phonation. This word comes from the Greek phōnē, voice, of course.
Special thanks today to Wikipedia for knowing all this stuff.