Smart thermostat
Smart thermostats are home automation devices responsible for controlling a home's heating, and sometimes air conditioning. They allow the user to control the temperature of their home throughout the day using a schedule, such as setting a lower temperature at night. As they are connected to the Internet, they allow users to adjust heating settings from other internet-connected devices, such as smartphones. This allows users to easily switch off the heating or air conditioning when the house is empty. This ease of use is essential for ensuring energy savings: studies have shown that households with programmable thermostats actually have higher energy consumption that those with simple thermostats, because residents program them incorrectly or disable them completely.[1][2]
Learning thermostats
Some smart thermostats can automatically learn when the house is likely to be occupied, and when it is likely to be empty. This allows it to automatically pre-heat or pre-cool the house, so that it's at a comfortable temperature when a resident arrives. If the residents' lifestyle changes, these smart thermostats will gradually adjust the schedule, maintaining energy savings and comfort. The Nest thermostat is a well known example.
Zoned systems
Rather than controlling the temperature of the whole house, zoned systems can control individual rooms. This can increase energy savings, for example by heating or cooling only a Home-office and not the bedrooms and other areas that are empty during the day.
Learning zoned systems
The most advanced smart thermostats combine both: they are able to learn when each room of a house is normally occupied and automatically schedule the heating for that room appropriately. Currently, the only such system available for home use is Heat Genius.