The IPA provides 5 click symbols: labial, dental, alveolar, palate-alveolar and lateral. These symbols specify the anterior release of these sounds. In order to indicate voicing the symbol for the voiced or voiceless velar plosive is added (clicks are essentially double articulations). Nasal clicks are transcribed by adding the symbol for the velar nasal.

The phonemic use of clicks is quite rare in languages of the world: they are restricted to languages in southern Africa. Clicks occur in all three groups of Khoisan language families and in the neighbouring Bantu languages which borrowed some of them (Zulu, Xhosa, Sotho, Ndebele). In addition to this there are three languages with clicks in East Africa (Hazda, Sandawe and Dahalo). It is generally assumed that clicks in the Bantu languages have been borrowed from Khoisan languages as a result of language contact. 

The video adjacent is an X-ray recording of some of the Zulu clicks by a female native speaker who reads a short Zulu sentence 'Iqaqa lega umgwaqo namaqanda ezinkukhu aqhuma athi qhu' (A polecat crosses the road with eggs of chicken and they burst making a qhu sound).

References:

Maddieson, I., Ladefoged, P., Sands, B. (1999). Clicks in East African Languages. In African Mosaic: Festschrift for J.A. Louw. Pretoria: University of South Africa Press. 59-91.

Wright, M. (2011). On clicks in English talk-in-interaction. Journal of the International Phonetic Association, 41/2, pp. 207-229.

LABIAL •
Voiceless
DENTAL •
Click
Phonetic symbol for the voiced dental click.
Phonetic symbol for the voiceless labial click.
Phonetic symbol for the voiceless dental click.
ALVEOLAR •
Phonetic symbol for the voiceless alveolar click.
PALATO-ALVEOLAR •
Phonetic symbol for the voiceless palato-alveolar click.
Phonetic symbol for the voiced palato-aveolar click.
ALVEOLAR LATERAL •
Phonetic symbol for the voiceless alveolar lateral click.
ALVEOLAR •
Phonetic symbol for the alveolar nasal click.
• PALATO-ALVEOLAR
Phonetic symbol for the palato-alveolar nasal click.

Clicks are speech sounds which are produced by means of a lingual ingressive airstream mechanism: the back of the tongue makes contact with the roof of the mouth in the velar area and additionally there is a constriction further forward in the vocal tract. The sides of the tongue make contact with the palate too so that a small amount of air is trapped in the space between the tongue and the hard palate.

In producing the click, the centre of the tongue is drawn downwards so that the trapped air is rarefied creating negative pressure. Upon the release of the anterior constriction air rushes into the vocal tract as a result of the pressure difference between atmospheric pressure and the pressure inside the vocal tract. This creates a popping sound with the degree of intensity depending on the place of articulation. Ladefoged and Maddieson (1987) indicate that the location and release of the front closure is what determines the click type while the velar or uvular articulation and other accompanying properties are considered as a click’s accompaniment which describe “properties of the back oral articulation together with any pulmonic activity, laryngeal setting, or nasal airflow that accompanies (sic) the click” (Ladefoged & Maddieson, 1987: 247).

Symbols for the phonetic transcription of clicks

Click
Voiceless
Click
LABIAL •
NASAL CLICK
Phonetic symbol for the labial nasal click.
• DENTAL
Voiced
• LABIAL
Click
Phonetic symbol for the voiced labial click
Voiceless
Voiced
• DENTAL
Click
Voiceless
Click
Voiced
• ALVEOLAR
Click
Phonetic symbol for the voiced alveolar click.
Voiceless
Click
Voiced
• PALATO-ALVEOLAR
Click
Voiced
• ALVEOLAR LATERAL
Click
Phonetic symbol for the voided alveolar lateral click.
NASAL CLICK
NASAL CLICK
NASAL CLICK
ALVEOLAR
LATERAL
ALVEOLAR
• VOICED
VOICED 
LABIAL
VOICELESS •
• VOICELESS
NASAL •
DENTAL
PALATO
ALVEOLAR
• NASAL
ALVEOLAR LATERAL •
NASAL CLICK
Phonetic symbol for the alveolar nasal click.
 
 
 
 
 
 
 
 

Nutshell phonetics of clicks