a fricative is a consonant produced by forcing air through a constricted space. In other words, in producing these sounds, turbulence is caused when the air is forced trough a smaller opening. Depending on which parts of the vocal tract are used to constrict the airflow, that turbulence causes the sound produced to have a specific character (say have very slowly and stretch out the /v/; pay attention to what happens to the air when the teeth touch the bottom lip for the /v/). There are five types of fricative in English. For an interactive example of each sound (including descriptive animation and video), click this link, then in the window that opens, click fricative, and select the appropriate sound.