Housing consists of four main category. Unskilled Workers, Skill Workers, Executives, Elites. Each category is then divided into four more categories. Low density, Medium density, High density, and the special Exceptional.
Unskilled Workers are the first type of housing unlocked, right after the city hall is built. They are needed in the Food Industry, Heaving Industry, Manufacturing, Retail, Hotels, Services and Utilities.. They have very basic needs for services and environment, only needing the service to be present, and livable environment quality.
Skilled Workers form the backbone of a full grown city. They are unlocked along side Unskilled Workers, and are needed in Medium density and up in Heavy Industry, and Low density of everything else. Although slightly pickier than Unskilled Workers, Skilled Workers require only a minimum of services, and only a slight increase in environmental quality.
Executives are much more pickier than Skilled workers, needed in most higher end businesses. They require good environmental quality, and simply having a service in the area will not be enough. Instead, they will need multiple types of each services, such as a basketball court, a swimming pool, and an indoor polo game all covering the area. They refrain from taking most public transport, only doing so in dire circumstances. Holiday choices will influence their happiness, but to a less extent than elites.
Elites are just that, elites. And they know they are, so they demand the best of everything, pollution, and lack of services, including in diversity, will push these citizens away, making the largest companies lose their managers. A good transportation system (roads of course, elites of their own chauffeurs) that leads away from pollution is a must, and holiday zones are most certainly necessary.