Sounds with vocal chord vibration are voiced and sounds without vocal chord vibration are voiceless.
Contrast the sounds of the letters v and f. Place your fingers on your throat and make the sounds. You should feel a vibration when you make the v sound and no vibration when you make the f sound. V is voiced and f is voiceless.
The Voiced Sounds in English
b d g j l m n ŋ(ng) r v z ð(th in the) and all vowels
The Voiceless Sounds in English
ch f h k p s sh t θ(th in think)