Stop; If the active and passive articulators actually touch, stopping airflow through the oral cavity completely for a brief period, the sound articulated is a stop. If you put your lips together to produce [p] pea, and hold them in that position, you will feel the build-up of air which is then released when you move from the stop to the following vowel. Further back in the vocal tract, [t] tea and [k] key are also stop sounds. More accurately, all these are plosives, the term for oral stops produced on a pulmonic egressive airstream, just as clicks are stops produced on a velaric ingressive airstream, for instance. Plosives may be voiceless, like [p], [t] and [k], or voiced, like their equivalents [b], [d] and [m]. The active articulator touches the passive articulator and completely cuts off the airflow through the mouth. English stops include: [p], [d], [k], [m].
Fricative; During the production of a fricative, the active and passive articulators are brought close together, but not near enough to totally block the oral cavity. This close approximation of the articulators means the air coming from the lungs has to squeeze through a narrow gap at high speed, creating turbulence, or local audible friction, which is heard as hissing for a voiceless fricative, and buzzing for a voiced one. English [f] five and [s] size are voiceless fricatives, while [v] five and [z] size are voiced. The active articulator doesn't touch the passive articulator, but gets close enough that the airflow through the opening becomes turbulent. English fricatives include [f], [v], [z].
Approximant; It is relatively easy to recognise a stop or fricative, and to diagnose the articulators involved, since these are either touching or so close that their location can be felt. In approximants, on the other hand, the active and passive articulator never become sufficiently close to create audible friction. Instead, the open approximation of the articulators alters the shape of the oral cavity, and leads to the production of a particular sound quality. There are four approximant consonant phonemes in English: /j/ yes, /w/ wet, /r/ red (although as we have seen, /r/ may have a tapped allophone for some speakers) and /l/ let. All these approximants are voiced. The active articulator approaches the passive articulator, but doesn't even get close enough for the airflow to become turbulent. English approximants include [j], [w], [r], and [l].
Affricative; The subclass of affricates consists of sounds which start as stops and end up as fricatives; they behave as single, complex sounds rather than sequences. Stops generally involve quick release of their complete closure; but if this release is slow, or delayed, the articulators will pass through a stage of close approximation appropriate for a fricative. The two relevant sounds for English are [tʃ], at the beginning and end of church, and its voiced equivalent [d_], found at the beginning and end of judge. If you pronounce these words extremely slowly, you should be able to identify the stop and fricative phases. Affricates can be seen as a sequence of a stop and a fricative which have the same or similar places of articulation. They are transcribed using the symbols for the stop and the fricative. If one wants to emphasize the affricate as a "single" sound, a tie symbol can be used to join the stop and the fricative (sometimes the fricative is written as a superscript).
Nasal; The soft palate can be lowered, allowing air to flow out through the nose, or it can be raised to block nasal airflow. As was the case with the vocal cords, what the soft palate is doing is independent the other articulators. For almost any place of articulation, there are pairs of stops that differ only in whether the soft palate is raised, as in the oral stop [d], or lowered, as in the nasal stop [n]
Lateral; When you form an [l], your tongue tip touches your alveolar ridge (or maybe your upper teeth) but it doesn't create a stop because one or both sides of the tongue are lowered so that air can flow out along the side. Sounds like this with airflow along the sides of the tongue are called lateral, all others are called central (though we usually just assume that a sound is central unless we explicitly say it's lateral).
The side of the tongue can lower to different degrees. It can lower so little that the air passing through becomes turbulent or it can lower enough for there to be no turbulence (a lateral approximant). The [l] of English is a lateral approximant.