Low-voltage thermostats can be programmable or non-programmable. Most thermostats sold in the United States are considered low-voltage thermostats. Line-voltage thermostats are more popular in other parts of the world.
Most low-voltage thermostats draw 24 volts of electricity, although some models can use as little as 6 volts or as much as 30 volts of electricity. The electricity used by the heating unit is drawn from a separate electrical connection. Line-voltage thermostats, on the other hand, use the same electrical line as the heating unit they're connected to. As a result, they must be capable of handling 120 volts or 240 volts, so they typically have significantly thicker wiring.