Manual - Conventional dial or lever style of mechanical thermostat. It doesn’t have many features.
Digital - This has a display screen and is often capable of a variety of useful functions.

The number of terminals on the thermostat that can be used to individually control separate heating functions. Many furnaces that are called two stage heaters will require at least a two stage thermostat to operate appropriately. You may also choose a multiple stage thermostat when pairing different heating equipment. Such as a heat pump and a furnace.

The number of terminals on the thermostat that can be used to individually control separate cooling functions. Some of the higher efficiency air conditioners and heat pumps are designed to use multiple stages of cooling.

Indicates that this thermostat is compatible with heat pumps.

Indicates if the thermostat operates electrically on batteries or power from the furnace/air handler.

A "2-wire Millivolt" system. These are usually gas-fired wall or floor heaters that do not have forced-air blowers. In other words, they are convection heaters.

Indicates that this thermostat can be used with either a gas furnace or oil furnace.

Indicates that this thermostat is compatible with electric furnaces or air handlers with electric heater coils. The voltage to the thermostat will be the normal 24 VAC. This feature is different from a "Line Voltage Capable" thermostat which operates with 120 to 240 VAC.

Allows for you to establish by schedule when you want your furnace/air handler fan to turn on and off automatically to circulate home air, even without your furnace or air conditioner operating.

Brings the room temperature back to the programmed temperature at the set time, even when you may have manually over-ridden the program in the interim which will maximize comfort and energy savings.

Allows easier viewing of disply screen in the dark.

When used with an outdoor air sensor, this features allows the thermostat to sense the outdoor temperature and indicate such on the screen. Also allows some thermostats to automatically decide whether to heat or cool the home based on outside temperatures.

Some thermostats provide for wiring to an accessory called an indoor sensor. The indoor sensor mounts in the home living space away from the thermostat, and tells the thermostat what the temperature is in that area. The thermostat will then use the temperature from the sensor to indicate the home's current comfort level, instead of just the temperature near the thermostat.

This feature allow a user to control from the thermostat any other electronic device such as an independent fan.

This feature allows the thermostat to automatically switch from heating to cooling based on the temperature inside. This is useful between seasons when heating may be needed at night, and cooling during the day.

Having this feature will store your set temperature or temperature schedule. In the event of a power failure, you won't have to reprogram the thermostat when the power returns.

Reminds you that it's time to change your furnace or air handler filter.

Reminds you that it's time to change your humidifier pad, which is normally done so annually.

Allows you to control your whole-house humidifier from your thermostat.

Allows you to program your thermostat to operate your heating/cooling system at a minimum when you leave on vacation (to save money), and then automatically return the temperature to a normal comfort setting just before you arrive home. Having this capability will mean that when you return, your house will be nice and comfortable.

Some units can be accessed remotely from a touch-tone phone or web interface.