Buccal consonants are pronounced from the velum onwards; this comprises all labial, coronal and dorsal places of articulation. As such, they are a class of "typical" consonants. It contrasts with guttural (comprising the radical and glottal POAs).
Debuccalization refers to the change from a buccal consonant to a glottal. Typically stops will end up as [ʔ], fricatives as [h], though there is room for variation (eg. *ɟʱ, *gʱ → [ɦ] in Sanskrit). The opposite change is buccalization; typically the result will be a velar (eg. *ʔ → k, *h → x). (The term glottalization is usually only applied to changes resulting in [ʔ] or glottalized consonants.)
The buccal/guttural division is not applied to vowels.