A fricative is a consonant produced by forcing air through a narrow channel made by placing two articulators close together. These may be the lower lip against the upper teeth, in the case of [f]; the back of the tongue against the soft palate in the case of German [x] (the final consonant of Bach); or the side of the tongue against the molars, in the case of Welsh [ɬ] (appearing twice in the name Llanelli). This turbulent airflow is called frication.
A particular subset of fricatives are the sibilants. When forming a sibilant, one still is forcing air through a narrow channel, but in addition, the tongue is curled lengthwise to direct the air over the edge of the teeth. English [s], [z], [ʃ], and [ʒ] are examples of sibilants.
The usage of two other terms is less standardized: "Spirant" is an older term for fricatives used by some American and European phoneticians and phonologists. "Strident" could mean just "sibilant", but some authors include also labiodental and uvular fricatives in the class.
The airflow is not completely stopped in the production of fricative consonants. In other words, the airflow experiences friction.
[s] voiceless coronal sibilant, as in English sip
[z] voiced coronal sibilant, as in English zip
[s̪] voiceless dental sibilant
[z̪] voiced dental sibilant
[s̺] voiceless apical sibilant
[z̺] voiced apical sibilant
[s̟] voiceless predorsal sibilant (laminal, with tongue tip at lower teeth)
[z̟] voiced predorsal sibilant (laminal)
[s̠] voiceless postalveolar sibilant (laminal)
[z̠] voiced postalveolar sibilant (laminal)
[ʃ] voiceless palato-alveolar sibilant (domed, partially palatalized), as in English ship
[ʒ] voiced palato-alveolar sibilant (domed, partially palatalized), as the si in English vision
[ɕ] voiceless alveolo-palatal sibilant (laminal, palatalized)
[ʑ] voiced alveolo-palatal sibilant (laminal, palatalized)
[ʂ] voiceless retroflex sibilant (apical or subapical)
[ʐ] voiced retroflex sibilant (apical or subapical)
All sibilants are coronal, but may be dental, alveolar, postalveolar, or palatal (retroflex) within that range.