One way to document your understanding of customers is to compose profiles or personas. A profile describes the situation, problems, and general personality traits of one type of user or buyer in the market for your product. A persona differs from a profile in its level of specificity.
Personas are a bit like what some people speculated Deep Throat was in Woodward and Bernstein's account of Watergate, All the President's Men: a composite of various sources. (We now know that Deep Throat was a single person, Mark Felt). According to the composite character theory, Woodward and Bernstein concocted the character Deep Throat to represent a combination of many sources that gave Woodward guidance and information in their investigation of Watergate.
Personas are similar in that they represent all of the people in a market segment as a single person. That single person has very specific characteristics that many of the people in the segment do not have, but that nonetheless serve as representative examples of the types of situations and problems they face.
Thus profiles more comprehensively and accurately describe the range of people in a market segment, but a set of personas provides a more concrete picture of the types of people in the segment.