StopsSpeech segments produced with a complete closure at some point in the vocal tract behind which the air pressure bluids up and can be released explosively.
/p, b, t, d, k, g/
FricativesIn the stop [t], the tongue tip touches the alveolar ridge and cuts off the airflow. In [s], the tongue tip approaches the alveolar ridge but doesn't quite touch it. There is still enough of an opening for airflow to continue, but the opening is narrow enough that it causes the escaping air to become turbulent (hence the hissing sound of the [s]). In a fricative consonant, the articulators involved in the constriction approach get close enough to each other to create a turbluent airstream. The fricatives of English are /f, v, θ, ð, s, z, ʃ, ʒ, h/
ApproximantsIn an approximant, the articulators involved in the constriction are further apart still than they are for a fricative. The articulators are still closer to each other than when the vocal tract is in its neutral position, but they are not even close enough to cause the air passing between them to become turbulent. The approximants of English are /w, j, r/
AffricatesAn affricate is a single sound composed of a stop portion and a fricative portion. In English [tʃ], the airflow is first interuppted by a stop which is very similar to [t] (though made a bit further back). But instead of finishing the articulation quickly and moving directly into the next sound, the tongue pulls away from the stop slowly, so that there is a period of time immediately after the stop where the constriction is narrow enough to cause a turbulent airstream. In [tʃ], the period of turbulent airstream following the stop portion is the same as the fricative [ʃ]. English [dʒ] is an affricate like [tʃ], but voiced.
LateralsPay attention to what you are doing with your tongue when you say the first consonant of [lif] leaf. Your tongue tip is touching your alveolar ridge (or perhaps your upper teeth), but this doesn't make [l] a stop. Air is still flowing during an [l] because the side of your tongue has dropped down and left an opening. (Some people drop down the right side of their tongue during an [l]; others drop down the left; a few drop down both sides.) Sounds which involve airflow around the side of the tongue are called laterals. Sounds which are not lateral are called central.
[l] is the only lateral in English. The other sounds of Englihs, like most of the sounds of the world's languages, are central.
More specifically, [l] is a lateral approximant. The opening left at the side of the tongue is wide enough that the air flowing through does not become turbulent.
Speech segments during whose production the velum is lowered closing the entrance to the oral cavity, the air being allowed to escape through the nose. /m, n, ŋ/