You need to know everything obtainable about your customers. This list will get you started. You will need their age, sex, marital status, income levels, education levels, geographic locations, where they shop, if they go to church, if they have kids, ages of children, political affiliation, what they like about your company, what they don't like, why they shop with you, what they value about your products/services, what they like the most/least about products/services, and whether prospects are unaware of your brand or products/services. Customers want you to know certain things about them and if you do, they will remain loyal to you as you take time to know them.
Customer Segment _ this is talked about most of the time, but let's accept the fact that today's customers are fragmented to smaller niches. Your product and service will not be consumed by only one segment and at critical mass. it is likely to be consumed by smaller segment and multi. No more critical mass, but rather a solid spread of niches. For example, who buys iPod's today? If Apple only think about yuppies they are making a big mistake because apparently iPod is also used by women and people with active lifestyle. Another example, who buys from hypermarket? If hypermarket only focuses on households they may not survive or be successful like today. Hypermarket main segment is actually small business restaurant operators that buy in larger quantity compare to households and on daily basis. I've been there, I know! LOL