Here is a quick overview of the 6 pairs of voiced and their counterparts, voiceless (devoiced) consonants.

Vocal chords vibrate (VOICED): Б, В, Г, Д, Ж, З (b, v, g, d, zh, z)
Vocal chords do not vibrate (VOICELESS): П, Ф, К, Т, Ш, С (p, f, k, t, sh, s)

1. Word final devoicing
Voiced consonants at the end of words are pronounced as voiceless (including soft sign as ending):

We write -> We pronounce:
гараж -> гара[ш] (garage)
маркетинг -> маркетин[к] (marketing) 
лебедь -> лебе[т]ь (swan) 

2. Voiced-voiceless assimilation
When voiced and voiceless consonants are adjacent to each other, the nature of the second consonant dictates the nature of the first:

voiced + voiceless -> voiceless + voiceless
субтитры -> су[пт]итры (subtitles)

voiceless + voiced -> voiced + voiced
баскетбол -> баске[дб]ол (basketball) 

Check out this video: http://bit.ly/1525p8J for more information. 

Please let me know if you have any questions - I am here to assist!

Спасибо!
Наташа