What is a User Profile?
User profiling is a process of collecting user research, analyzing the data, and developing personas. Personas are detailed descriptions of imaginary people constructed out of well-understood, highly specific data about real people. The goal of user profiling, of creating personas, is to make UCD possible.
Persona definition on Wikipedia
Why are personas needed?
"Interaction design is a complex and difficult craft and requires good tools like any other. The popularity of personas has exploded because they are the foundational tool upon which the practice of interaction design rests. Interaction design is about making a particular group of humans effective at achieving a narrow set of goals. Because using personas is a remarkably powerful technique for bringing those humans and their objectives into focus, it becomes the most critical tool for designing the behavior of software." - Alan Cooper
"No matter what we are designing, building, or helping to build, we want our products (including software, hardware, consumer goods, and services) to be useful, appreciated, and profitable. We want to help create products quickly and cost effectively, but with the right set of features and good quality. We want these products to hit the market and instantly inspire demand, desire, and loyalty. We want people to use our products repeatedly and happily, encountering just the right functions at the right times and finding that the products grow with them as they develop expertise. We want our efforts to result in products that delight people, and to delight people we have to have some idea of who these people are and what they want." - The Persona Lifecycle