Paraphrasing Webster more clearly, a fricative is a
speech sound where your mouth is
constricted at some point, enough to cause audible
friction, but not enough to
stop the breath completely.
The fricatives are classified mainly by place of articulation -- where in the mouth the constriction occurs -- and by whether they are voiced or not. For most languages this is enough to specify them. Moving from front to back of the mouth, they are:
- bilabial f and v are relatively rare: Japanese f is bilabial, as is Spanish b or v between vowels, though the friction there is very weak.
- labiodental f and v as in English.
- dental th as in thin and th as in then. These sounds also occurs in for example Icelandic, Greek, (Castilian) Spanish, Arabic, and Burmese, but are not very common worldwide.
- alveolar s and z.
- postalveolar (palato-alveolar) sh as in English, and the zh sound in vision, sabotage: the sh sound occurs commonly enough in the world's languages but the latter is a bit rarer; French, Portuguese, and all the Slavonic languages have it.
- alveolo-palatal sh and zh are rarer, but occur in Chinese and Polish: in Polish they are written with acute-accented ś and ź, and contrast with the palato-alveolar sz and rz. In (standard) Chinese the pinyin x is alveolo-palatal. They are "softer" or thinner sounds than the English.
- retroflex sh and zh are quite rare, but are the Chinese sounds written sh and r in pinyin; and in Swedish and Norwegian the letters rs represent a retroflex sibilant.
- palatal ch as in German ich 'I', Reich; or the Greek okhí 'no'. It is a thin sh-like sound. The voiced equivalent is rare, a strong y sound, but it's the Icelandic g before i, as in the names Gísli, Egill.
- velar ch as in Achtung, loch, chutzpah, same as the kh of Arabic or Greek. This doesn't occur in standard English (except Scots) but is fairly common worldwide. The voiced sound gh is much rarer but occurs in Arabic.
- uvular kh and gh are rarer: Inuktitut and North Caucasian languages have them. However, the French r is uvular, normally voiced, but voiceless at the end of words like maître. Dutch g and ch are uvular.
- pharyngal h and ` are extremely rare, occurring in Arabic, and some related languages like Somali (where they are written x and c) and some accents of Hebrew, and also in North Caucasian languages. See Arabic pronunciation for my fairly futile attempts to explain them more closely.
- glottal h is normal voiceless, but between vowels as in behind it is voiced in English. In some languages, such as Arabic and Czech, it always has this voiced sound. I've not come across a language that has both sounds in contrast.
Whereas almost every language in the world has at least some of the normal fricatives described above (
Australian Aboriginal languages haven't any), mechanisms other than normal constriction are quite rare in the world.
First up are the lateral fricatives, where the tongue constricts to cause friction on only one side. Welsh has a voiceless palatal lateral fricative ll. Southern African languages such as Xhosa and Zulu have dental lateral fricatives hl and dhl. North Caucasian languages typically have huge numbers of consonants, and some of them have lateral fricatives too.
Fricatives are not usually subject to different kinds of phonation, unlike stops, but both Korean and Burmese have an aspirated s: that is a hissing sound something like the s+h in mishap.
In languages that have coarticulation such as labialization or palatalization, fricatives are subject to these: for example Irish and Russian have f and fy sounds, and likewise palatalized forms of most of their other fricatives. North Caucasian languages typically have both kh and labialized khw and other such pairs.