Thermal energy is the internal energy of a body (solid/liquid/gas). It means something like potential energy. The thermal energy is directly proportional to the temperature of the body. It can be assumed that the thermal energy is directly proportional to the temperature of the body. During boiling and melting thermal energy increases without change in temperature.
Thermal and PotentialEdit
To explain Thermal Energy we can take a simple example of the potential energy. Let us assume a ball is kept on first floor. So it has internal energy (because of this internal energy it can fall upto ground level). This internal energy is let us say = 1kcal. Then we take that ball to 5th floor and now it has internal energy = 5kcal. Now this increase in internal energy is due to storage of energy which was spent to take ball from floor 1 to 5. Also to recover this internal energy ball needs to fall upto ground floor.
Now in thermal energy parlance let's say some water is at temperature 15oC it has energy equal to let's say 150Kcal. Now, if we heat it to 30oC the thermal energy becomes 300Kcal. We can now extract this delta thermal energy of 150Kcal by cooling water to 15oC. This essentially means that energy used to heat the water is only extracted.
Advantages and DisadvantagesEdit
It is one of the easiest way to store the energy. However to retain this energy the storage needs to be tightly insulated from the surrounding since it will lose the stored energy because of cooling due to air.
If we are using this energy for heating purpose directly it is best suited. However due to Thermodynamic limitation it becomes impossible to convert total thermal energy into the usable mechanical / electric energy.