There are many types of thermostats, from the most basic to the tech savvy. Thermostats let your HVAC unit know how long it must run in order to reach the desired temperature that you want in a space. For example, if the space is 72 degrees, and you want it to be 68 degrees, it will run until the thermostat sensor reads that it is 68 degrees in that space.
The most basic thermostat will simply control whether you want to heat or cool your space, whether you want your fan running and the temperature that the room or space should be. From there you jump to programmable thermostats. These thermostats allow you to do everything the basic thermostat can do as well as program when you want your HVAC to run and at what temperature. For example, if you are at work all day, you can set it to keep the space at a different temperature and then have it rise maybe an hour before you get home. This saves you money because you will not be conditioning a space that no one is using. There are many levels of programmable thermostats from the basic to touch screens. Many of these have other accessories or options like letting you know if a filter needs to be replaced, that your unit needs to be serviced or the ability to adjust your humidifier. The latest must have is a thermostat that connects to your Wi0fi and gives you the ability to control remotely from a smart phone or a computer.