The basic principals of an automatic irrigation system are fairly simple. There are some variations on the actual layout and it can be a bit overwhelming trying to figure out exactly where things are located since they are often buried. While this is an oversimplification, in general the water comes from a source. If it is a domestic water supply, a device to prevent cross contamination is required. The water then goes to a manifold where it is divided into zones. Each zone is controlled by an electronic valve that is operated from the controller. Usually the manifold and controller are fairly close together with the control module located in a protected location, often inside the house or garage. There will be individual "hot" wires running from the controller to each of the valves and a single common or ground wire daisy chained to each valve. The controller is a timer that opens and closes each valve according to the time you program into it. From the valves, pipes are run to create the zones. The number of zones will depend on the gpm available and the amount of water required be the sprinklers in each zone. So, there is some math involved because the size of pipe and water requirements of the sprinklers in each zone determined how many zones are needed. When laying out a new system, I always recommend making a scale drawing of the property including trees and shrubs, finding out the amount of water available, size of the water meter (if appropriate), and pressure of the water source. Then take that information to an irrigation supply company and have them lay out what you should have. This service is sometimes free if you agree to buy all materials from them, other times a fee is charged, but it is well worth it. In your case, the system apparently is already in place. If the controller is missing, it is likely the wires are still there. Controllers an not expensive and are quite easy to install especially if just replacing an old or missing one.