In a thermal plasma a gas is heated such that the electrons have sufficient energy to separate from nuclei leaving bare nuclei. Thermal plasma are thus highly charged and highly conductive.
A plasma is classified as thermal if the assumption of local thermodynamic equilibrium (LTE) holds. This means that all the plasma properties are only function of thermodynamic quantities such as pressure and temperature, all the internal distributions of atoms and molecule follow the Boltzmann one and the electron energy distribution (EEDF) is a Maxwellian.
On the contrary, a plasma is defined as non-thermal when the LTE is not verified. Often in this case the distributions can non-Boltzmann and must be calculated with the so called state-to-state approach, while the EEDF must be determined through the solution of the Boltzmann equation.
Thermal plasma are usually obtained in high (atmospheric) pressure conditions. However, to establish if a plasma is thermal or not is a matter of characteristic times. If the gradients (in space and time) are small with respect to variations due to collisions, than the plasma can be considered in LTE. This is the case of plasma torches, lightning and so on. However, in DBD discharge, even if the pressure is quite high, the gradients are very large and therefore DBD'a are considered an high pressure non-thermal plasmas.
High pressure non-thermal plasmas are also present in the shock wave of hypersonic flows.