In phonetics, plosives (or stops) are speech sounds which are produced with a constriction of complete closure. This means that an active articulator (e.g. the tongue) touches a passive articulator (e.g. the roof of the mouth). This prevents the air from escaping via the mouth. At the same time the velum is raised so that the air cannot escape via the nose either and air pressure keeps building up behind the obstruction in the mouth. Upon release of the active articulator the air escapes with a audible plosion: this is the perceptual essence of these sounds. At the bottom of this page, there are several videos illustrating this type of articulation.
All the languages of the world have plosives. The first sounds in the English words "pea", "tea" and "key" are examples of plosives. The overall configuration of the tongue in the plosive [ t ] in "tea" is illustrated in the video on the right. It can be seen that both sides of the tongue and the tip of the tongue are raised to touch the roof of the mouth. This effectively stops the airflow in the mouth for the duration of the plosive.
Languages can have plosives at a wide range of places of articulation ranging from labial (anterior) to glottal (posterior). Most languages of the world have voiced and voiceless plosives at three places of articulation, i.e. labial, alveolar and velar. This creates excellent perceptual contrast between the plosives. Overall, voiceless plosives are substantially more frequent in languages than voiced plosives and arguably this has to do with the fact that voiceless plosives are somewhat easier to articulate.
In terms of the phonetic transcription of plosives, the phonetic notation system of the International Phonetic Association distinguishes between 14 discrete main phonetic symbols for plosives. Symbols for other plosives can be derived from these by using diacritics. Most phonetic symbols and some of the diacritics are illustrated in the videos above which you may find useful as ear training exercises in practical phonetics.
Some plosives have a stricture of complete closure at two different places of articulation: these are double articulations. In theory various combinations are possible but the labial-velar stop is particularly frequent. Although these sounds are referred to as "labial-velar" they are conventionally transcribed phonetically by a combination of the phonetic symbol for the velar plosive followed by the symbol for the labial plosive. Labial-velar plosives are particularly frequent in West and Central African languages.
The videos below illustrate the processes involved in the production of the voiceless and voiced plosives in Dutch. In all cases you can observe a full obstruction in the vocal tract, the raising of the velum, the increase in air pressure behind the obstruction and the plosive release of air after the release of the active articulator.
Visual illustration of a few plosive articulations
The vocal folds do not vibrate
The vocal folds do vibrate