Primarily, they're phonological classifications: we're treating the sound as an idealised, isolated phoneme, a basic element of the sound system. Thus, the articulatory classification of the phoneme /k/ is voiceless velar plosive.
But wondrous things happen when phonemes are actually phonetically realised. For example, while no one would deny that the 'k' sounds in the famous example "Keep calm, coronel" are allophones of the phoneme /k/, in phonetic terms the tongue position in each 'k' sound is influenced by the following vowel sound, being more front, back, and central respectively, and so the sound comes out a tad differently each time. For this reason, phoneticians on the ground employ a stupendous array of diacritics to indicate small phonetic differences.
You can confirm what I say with "Keep calm, coronel" by attempting to pronounce 'calm' with the 'k' sound of 'keep', i.e. you fix in your head that you're going to say 'keep' but actually say 'calm', and so on. Have fun.
- For Teachers