What is a Brand Key?

Really, one of the best questions I read here :) Thinking about a brand as a real person, who has personality, DNA, image...etc and thinking of your client as a friend of this brand; then why your friend choose you rather than all people in the world to give you his friendship "which is loyalty when talking about client"? The real world fact is: people are emotionally driven than rationally driven, the same fact is applied on your client, what makes your client loyal to your brand the same as your best friend ?! and how does he see your brand "emotionally" as well?! the answers for both questions are your brand keys.
You will be amazed that those drives can be your product colors, planogram "in retail industries", your team since of humor "in service oriented companies"....etc the combination of those drives are your brand keys.

http://en.wikipedia.org/wiki/Brand_keys

Very aptly put Hossam!

It's about uncovering what makes customer loyalty happen, providing the psychologically-based metrics that accurately predict future consumer behavior in a category.

A brand key is a guideline of sorts that helps you define what a brand stands for, the framework behind it and the communication that flows from the brand. It is the starting point of a brand-building exercise; the point which shows the way forward when launching a new brand or reshaping an existing one. All in all, a brand key is a set of strategic ideals that together defines the tone of voice a brand uses to communicate and emote with stakeholders, internal and external.