1. Thermodynamic equilibrium
For attaining a state of thermodynamic equilibrium, the following three types of equilibrium states must be achieved.
Thermal Equilibrium: The temperature of the system does not change with time and has same value at all points of the system.
Mechanical Equilibrium: There are no unbalanced forces within the system or between the surroundings. The pressure in the system is same at all points and does not change with respect to time.
Chemical Equilibrium: No chemical reaction takes place in the system and the chemical composition which is same throughout the system does not vary with time.

2. If two bodies at the same temperature are in contact, there is no net heat flow from one body to the other. This condition is known as thermal equilibrium.

3. Heat Exchange between two system
System Boundary
Thermal Equilibrium
Thermal insulation
System A
System B tA tB
Heat Exchange between two system

4. Consider system A at uniform temperature tA and system B at another uniform temperature tB (tA > tB)
Let two systems be brought into contact and be thermally insulated from their surroundings but not from each other.
Energy will flow from system A to system B because of temperature difference (tA - tB), greater the temperature imbalance the higher would be the rate of energy transfer.
This will cause a reduction in the molecular activity (hence internal energy) of system A and a corresponding increase in molecular activity (and hence internal energy) of system B.
This energy flow continues till both systems attain the same temperature level at some intermediate value between their original temperature.
The two systems are then said to be in a state of thermal equilibrium.