Describing consonants

What makes one consonant different from another?

Producing a consonant involves making the vocal tract narrower at some location than it usually is. We call this narrowing a constriction. Which consonant you're pronouncing depends on where in the vocal tract the constriction is and how narrow it is. It also depends on a few other things, such as whether the vocal folds are vibrating and whether air is flowing through the nose.
We classify consonants along three major dimensions:

• place of articulation
• manner of articulation
• voicing

The place of articulation dimension specifies where in the vocal tract the constriction is. The voicing parameter specifies whether the vocal folds are vibrating. The manner of articulation dimesion is essentially everything else: how narrow the constriction is, whether air is flowing through the nose, and whether the tongue is dropped down on one side.
For example, for the sound [d]:

• Place of articulation = alveolar. (The narrowing of the vocal tract involves the tongue tip and the alveolar ridge.)
• Manner of articulation = oral stop. (The narrowing is complete -- the tongue is completely blocking off airflow through the mouth. There is also no airflow through the nose.)
• Voicing = voiced. (The vocal folds are vibrating.)