An obstruent is a speech sound such as [k], [d], or [f] that is formed by obstructing airflow. Obstruents contrast with sonorants, which have no such obstruction and so resonate. All obstruents are consonants, but sonorants include both vowels and consonants.
Obstruents are subdivided into plosives (oral stops), such as [p, t, k, b, d, ?], with complete occlusion of the vocal tract, often followed by a release burst; fricatives, such as [f, s, ?, x, v, z, ?, ?], with limited closure, not stopping airflow but making it turbulent; and affricates, which begin with complete occlusion but then release into a fricative-like release, such as [t] and [d].
Obstruents are prototypically voiceless, but voiced obstruents are common. This contrasts with sonorants, which are prototypically voiced and only rarely voiceless.