All interfaces are created for specific audience. In order to create a user friendly environment, designers have to understand who they create an interface for. The easiest and most effective way is to create a persona, who will represent an average user of our product.
It is like writing a script for a movie and it is fun. During a process of creating such a character we have to decide about all details, such as: name, sex, age, occupation, marital status, temper etc. All facts matter! The next step is to put our hero into some difficult but real situation in which the person has to use our interface. By imaging how the character will act in such a circumstances and what kind of difficulties he/she can meet, designers learn how to design more user friendly interface for a particular project.
An example: we create a website, an eCommerce site to sell eyeglasses. Our character is an older, rich person with a vision problem, siting in his fancy apartment in Manhattan. Right away a good designers can assume: “hmm… big font is needed. However, since he doesn’t care about the price, a font size of this element can be nough small to don’t let him see it clearly ;). The client of ours sells preatty expensive eyeglasses.”