There is one major difference used in describing the criteria for classifying consonants. To do so, one must examine the forming of both consonants and vowels.
To form a vowel, the flow of air over the lips, mouth, and teeth is not obstructed or stopped in any way. On the other hand, consonants are formed as air is obstructed and/or stopped as it passes over the lips, mouth and teeth.
There are six different ways to form consonants: bilabial, dental, alveolar, palatal, velar, and glottal.
Bilabial (two lips) consonant sounds are produced by placing the lips together. Consonant sounds produced through this action are /b/, /m/, and /p/.
Dental (teeth) consonants are formed by placing the tongue against the upper teeth. A consonant sound formed in this manner is /th/.
Alveolar consonant are formed by placing the tongue close to the top of the back of the top teeth and the roof of the mouth. Consonants formed in this way are /t/, /d/, and /n/.
Palatal consonants are formed when the tongue is raised against the hard palate on the roof of the mouth or moves toward the had palate. The consonant sound formed here is /j/.
Velar consonant sounds are formed by placing the back part of the tongue against the back part of the roof of the mouth. Consonant sounds formed here are /ng/, /k/, and /g/.
Glottal consonant sounds are formed at the glottis (the vocal cords and the spaces between the folds of the cords). The consonant sound produced is this fashion is /h/.