Have you ever seen sprinklers on in the rain? This can happen on any irrigation system that has a timer, but no control device.
There are a multitude of options available for preventing this scenario at your home. The controllers available today come in a wide range of prices, "techy-ness", and types. Hopefully this will help you select what control may work best for you.
The simplest type of device on the market is a Rain Sensor. Rain sensors wire into your existing timer and collect rainfall usually using disks made out of cork like material. These devices are the most inexpensive solution for preventing irrigation on a rainy day and are the easiest to install. When selecting a location to install your rain sensor, make sure the sensor is out in the open to receive rain (not under the roof or a tree for example). This is the only device that will work with mechanical timers.
Another control device is the Soil Moisture Sensor. These measure the actual amount of water in the soil and then prevent irrigation from turning on if it is not necessary. Based on studies, these devices have a greater potential for saving water than rain sensors, but they are also more expensive. The sensor should be buried in the driest area of the landscape (typically an area that has grass and full sun). The sensor needs to either be buried close to a zone valve or the irrigation timer.
The last type of control device is the ET Controller. These devices use weather information (temperature, rain, etc.) along with information about your landscape and irrigation system to determine how much water to apply and when to apply it. This may sound way too high tech, but some of these devices are actually user friendly. Some ET controllers use an on-site sensor to measure weather parameters and some receive a signal from the manufacturer with weather information. Signal-based controllers usually require a yearly service fee for signal reception. Below is a picture of a system with an on-site sensor for measuring temperature and rain.