Consonants may be voiced or voiceless, depending on whether they are pronounced with simultaneous vocal cord vibration, like all the consonants occurring in GA ago, already, mother, or with an open glottis, like those in GA post, fish, thick. The consonant in GA haw, which is produced with a narrowed glottis, is also classed as voiceless.
Consonants may also be partly devoiced, which means that there is vocal cord vibration during part of their articulation only. Examples would be the initial and final consonants of GA bed, guise, when these words are pronounced in isolation, in which case the voicing of the consonants may be confined to a very brief portion immediately before and alter the vowel.
The glottal stop is a special case: it is pronounced with a closed glottis and is therefore neither voiced nor voiceless.