Residential Zones are areas which you can set on the outline of your city's roads to signalize that the area is apt for constructing homes and moving in. Create green Residential zones to designate where Sims can build houses and move in. As soon as part of your city's parameters are labeled as a Residential zone, Sims automatically move in, but it’s up to you to keep them there. Laying down large blocks of green can mean a large population, but if you don’t have enough jobs, power, and utilities to keep all your residents happy, then a lot of those Residential buildings are going to be abandoned. Be smart about where you place homes, and be mindful of air and ground pollution to avoid creating a sick ward.
Remember - SimCity is a hyperrealistic game about city-management, so think as a normal resident of a city would and consider the conditions that your city has!