The type of thermostat you need can depend on what type of power your current system can/cannot supply to the thermostat.
My thermostat uses 24VAC common power
This means that your system provides a wire to your thermostat for constant power.
This wire is usually connected to the "C" terminal on your thermostat, and may be blue, brown, or black.
If choose this option, you will see thermostats that can be hardwired to your 24VAC common power.
My thermostat does not have 24VAC common power available
This means that your thermostat does not need a constant power supply from your system.
If you do not have 24VAC common power available, your thermostat selection will be limited to only those thermostats that can operate without 24VAC common power.
If you choose this option you will see only thermostats that will run without 24VAC (typically battery-powered or power-stealing).
You should choose this option if you are not sure, because any of the thermostats presented to you will run with/without 24VAC common power available.