The term "fashion" means clothing generally, and the study of it. Fashions vary considerably within a society according to age, man or woman, social class, generation, occupation and geography.
There is another term runway or also catwalk describes a narrow, usually elevated platform that runs into an auditorium, used by very good looking models to demonstrate clothing and modern new accessories during a fashion show.
A fashion week is a fashions industry event, lasting one week, which allows fashion designers, brands or houses to show and display their latest collections in runway shows and buyers to take a look at the latest trends. The most importantly is that it lets the industry know what's "in" and what's "out" for the season.
The most prominent fashion weeks are in the fashion capitals of the world: New York, London, Milan and Paris. Each city host a special week twice a year, because there are two major seasons per year - Autumn/Winter and Spring/Summer.