Difference between Heat and thermal energyWatch this thread
When you consider a monatomic ideal gas (for example) doing some isothermal expansion, the temperature remains constant, thus the thermal energy must remain constant for the gas. The gas does however do work while it expands, pushing on the outward pressure surrounding it. How does that work? How can it do work without losing this thermal energy associated with it? We say that there is a flow of thermal energy into the gas which goes into doing this work, and we call it heat to help make things easier.
I read on the internet that heat is thermal energy being transferred, why do we do need two different terms ?
So while the (thermal) energy is considered for the movements of gas particles, the heat is considered for their entropy. Two different things and that is why two different quantities!