You should select a few demographic factors that you think are relevant to your design. You only want to take time in an interview to ask a question if you are sure it will influence how a person will react to your concepts. The demographic information also allows you to keep a record of what type of user you were interviewing. For instance, if we were creating a page for the Society of Women Engineers, it would probably important to record the gender and the school of the person we were interviewing. Your client would want to know that you are basing design decisions on information from the target demographic group.
Basic needs are ones that are taken for granted in all products.
For example, all websites must be compatible with most browsers, they should not take too long to download, they should be easy to navigate, they should be easy to read, and so forth. Most users, in fact, have trouble articulating basic needs because they are assumed to be embodied in any good design. You don't have to ask a new car buyer if they want brakes, taillights, headlights, and windows. Differential needs, on the other hand, are ones that make a difference in a person's conscious evaluation of a product. The differential needs are the primary reasons that a person is using a product.