Enthalpy and its definiton I read books and i searched in internet to get what enthalpy is. It is equal to sum of internal energy and energy to keep its environment constant for movement.  It is 
    $ H=U+PV $  . But i found that it does not include p-v work. But H has in differential form $ dH=dU+PdV $. Here PdV must be p-v work. I am confused with these terms. What really enthalpy is and what is that it does not include p-v work?
 A: Let us consider heating a gas.
The heat (energy) that you put into the gas is equal to $\Delta H$
The increase in the internal energy of the gas is equal to $\Delta U$
And if the gas expands as you heat it then the gas does work - let us assume that this is at constant pressure $P$ -- the work done is $P \Delta V$
Conservation of energy means $\Delta H= \Delta U+P \Delta V $, which for s very small change can be written as $ dH=dU+PdV $.
So if a gas is heated at constant pressure the enthalpy delivered $\Delta H$ will partly heat the gas and partly do p-v work.
If a gas is heated at constant volume then the enthalpy delivered $\Delta H$ will only increase the internal energy of the gas because $\Delta V =0$. 
So the enthalpy required to heat gas at constant volume is less than at constant pressure because p-v work will be done when heating at constant pressure.
I hope this helps to figure out what enthalpy is. - I think $\Delta H$ can be summarized as the energy required for a process to happen (or if it is negative the energy released when a process happens).
