Zones
Alarm "Zones" refer to one or more sets of switches, contacts, sensors, etc. connected to a single alarm input. If a panel claims to have 8 hardwired zones, that means it has 8 connections for connecting the various "sensors" (switches, motion detectors, etc.). These are the primary inputs to the alarm system and are used to detect door and window activity in most alarms. It is entirely possibly to connect more than one sensor to 1 zone input. In fact you can connect all your doors and windows to 1 zone! However, the alarm panel uses these inputs for logging and display functions among other things. So if you connect everything to one zone, the panel can't tell the difference between the front door and an upstairs window. Most panels have the ability to tell you where the "violation" is, or has occurred. It does this by relating zones to to a specific set of sensors. Ideally, you would connect one sensor to each zone. This allows the panel to tell you which sensor is being tripped. A typical keypad has lights that relate directly to the zone inputs. If you wired the front door to zone 1, then you know the front door is open when the zone 1 light is on. It also provides basic troubleshooting information when sensors fail.
In some cases it may make sense to wire multiple sensors to one zone. If you have existing wires to one area of your home but more sensors than available wires, you may want to connect them all to the existing wires instead of running more wire. This is especially useful in areas that are difficult run wiring.
Parallel wired zones
Parallel wired zones are used with normally open (n.o.) zone connections. In this configuration, multiple sensors have the ability to start the flow and trip the alarm.
In the diagram above any of the 3 sensors (represented by the arrows and dots) could close and complete the circuit. Note the "no" and "c" designations. These are commonly seen on the panel zone inputs where no stands for Normally Open and '"c" stands for "common". See the page on no/nc for more information.
Series Zones loops
Series wired zones are normally used with normally closed sensors. In series wired zones the panel monitors the loop for a lack of current flow. All of the sensors are wired in "Series" one after the other. If any sensors changes to the open state, the current stops.
In the circuit above, any switch that opens will interrupt the flow of current.
All switches must be closed in order to allow current flow.