Nest is a new thermostat that learns. After installation, you tell Nest a few things and it begins to learn how and when you change the temperature.
After about a week, Nest has noted your schedule and will make adjustments based on that schedule. If you are away on Mondays, Nest will note and turn down the temperature. If you turn the heat or air down at night, Nest will adapt to that behavior and make sure it gets done, even if you forget.
You can also access Nest online or via an app on your smartphone and control the temperature remotely.