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.

Interactive, touch-sensitive display screen.

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.

The number days per week that the thermostat will be able to individually program. Ex. a 5,1,1 programmable thermostat will have the same program settings on Monday-Friday and will be able to individualize the programming on Saturday and Sunday. A 7 day programmable thermostat will be able to set a different programming schedule for each day of the week.

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.

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.

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

Will operate on 120 volts A/C or 240 volts A/C as indicated for that model. Electric baseboard and specialty heaters often require a line voltage capable thermostat.

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