Clicks are stops produced with two articulatory closures in the oral cavity. The pocket of air enclosed between the two closures is rarefied by a "sucking" action of the tongue. The release of the more forward closure produces a loud and extremely salient noise. This so-called velaric airstream mechanism is always ingressive (the air is sucked in) and can only be used for stops and affricates. Clicks are inherently stop-like or affricate-like depending on their place of articulation: clicks involving an alveolar or palatal closure are acoustically like plain stops, while bilabial, dental and lateral ones sound more like affricates.
Clicks are in all the Khoisan languages of southern Africa and in the neighbouring Nguni languages (Zulu, Xhosa, etc.) of the Bantu family, which borrowed them from Khoisan (there are some 80 languages in both groups). Clicks also occur in Sandawe and Hadza, two languages (or rather language groups, once believed to be branches of Khoisan) in Tanzania, Sesotho, spoken in South Africa and Lesotho, and in Dahalo, a South Cushitic language spoken in Kenya. The only non-African language known to employ clicks as regular speech sounds are Damin, an "alternative code" used by speakers of Lardil (Australia) -- actually an elaborate kind of language game. Of course "tut-tut" or "gee-up" noises can be used as meaningful interjections worldwide.
As noted above, clicks necessarily involve two closures: an anterior one which is regarded as primary and determines the click's place of articulation, and a posterior one which is typically velar or (less commonly) uvular. This posterior "accompaniment" can be transcribed as a velar or uvular oral or nasal. It's quite easy to pronounce a nasalised click if you realise that while maintaining the double oral closure you're free to breathe through the nose.
Since there are numerous (some of them really daunting) combinations of elements making up a click accompaniment, there are more than 100 ways of beginning a word with a click. These include a velar stop for basic clicks, a voiced velar stop, an aspirated one, a nasal one, a velar and glottal stop, a velar affricate, an ejective velar affricate, and others, as well. This means that pentagraphs like gk!x' are possible. The size of Khoisan click-phoneme systems ranges from 20 to as many as 83. In the latter language about 70% of words begin with a click; with the exception of Sandawe and Hadza, click languages permit only word-initial and word-medial clicks, never word-final.
In IPA transcription, the symbols ʘ, ǀ, ǁ, ǂ, and ! are used to represent bilabial, dental, alveloar lateral, alveolar, and retroflex clicks respectively. While there is no formalized ASCII standard for transcribing clicks, X-SAMPA (for eXtended SAMPA) does have a set of proposed symbols. They are not entirely satisfactory, as the alveolar lateral click is represented by a doubled dental click symbol, rather than a dedicated character or character-plus-backslash. It may make sense to either opt for ||\\, which uses the tradional IPA ||, or to use a mnemonic such as #\\ or "\\ - both unused, according to the summary chart at the end of http://www.phon.ucl.ac.uk/home/sampa/ipasam-x.pdf. However, such was not the choice of the author of the preceding document, for whatever reason.
