What is enthalpy?

The total heat content of a system at constant pressure is equal to the sum of the internal energy and PV. This is called the enthalpy of a system which is represented by H.
Note that enthalpy is also called as heat content.
H = U + pv
Enthalpy which depends on the three state functions: internal energy, pressure and volume.Hence it  is also a state function.
Enthalpy of a substance cannot be measured, but change in enthalpy can be measured.
Change in enthalpy =  Enthalpy of products - Enthalpy of reactants
∆H = HP-HR
From the first law of thermodynamics
∆U =qp-P∆V
qp =heat absorbed by the system
-P∆V=work done by the system.
For finite changes at constant pressure,       
∆H= ∆U+∆(PV)
∆H=∆U+P∆V         (∵ P is constant )         
∆H=qp            
∆H=-Ve for exothermic reactions
∆H=+Ve for endothermic reactions.