The Key Wardrobe is the person in charge of the wardrobe on a set. Depending on the budget of the project, the Key Wardrobe might serve as both the costumer and the person in charge of the wardrobe. They determine what a character wears and when depending upon the style of clothing that the director, writer or producers deem appropriate.
The duties of someone in wardrobe vary from project to project, but essentially the job consists of reading the script and based upon the scenes and character descriptions, the Key Wardrobe person will determine what style and types of clothing a given character will wear.
Once there is a basic understanding of what is needed, the Key Wardrobe will either have to shop for, or make the desired clothing. If the project calls for specialized clothing that can't be found in a local retail store, than the Key Wardrobe person will either have to make the clothing themselves, or contract a Costumer to design something specifically tailored to a given character.
The job entails taking measurements and tailoring all of the wardrobe to the actors; logging in all pieces of wardrobe and keeping track of them over the course of the production; and maintaining the wardrobe (dry cleaning, mending, etc.) throughout the desired time period.
You need to have an eye for fashion. Not your typical Milan style fashion, but it's more about being able to read a description of a character and determine the type of clothing that character would choose to wear. The best wardrobe people are those that make the wardrobe feel organic to the film or TV production. After all, you don't ever want to have the clothing be a distraction for the audience (unless of course that was the intention).
You should have a genuine love of clothes as well as shopping. More often than not, you will spend at least half of your time in retail stores (depending on the project), negotiating deals for your wardrobe as well as trying to find things that will make your characters feel unique and different, while remaining true to who the writer/director/producer envision.
What to Expect:
The best wardrobe people have spent years working in the field before becoming Key Wardrobe. It's essentially an apprenticeship and if you find someone willing to teach you the trade, then you are already that much further ahead.
The best wardrobe people command the highest salaries. Often they are able to create unique and interesting fashions that might even come to life OFF the screen.