what is an exact differential equation.............?

DEAR RAJ,

           EXACT DIFFERENTIAL EQUATION IS DIFFERENTIAL EQUATION WHICH IS THE PARTIAL DIFFERENTIATION IS EQUAL.THANK YOU.

@ raj 
diffrential equation is besically an equation which involving the derivative of a function . 
or in otherwords u can say , its a mathematical equation which relate some function with its derivative . 
and in application the function usually represent some physical quantity like rate of change , rate of increase and decrease etc . 
and IN THE CASE IF PARTIAL DIFFRENTIATION IS EQUAL THEN ITS CALLED AS AN EXACT DIFFRENTIAL EQUATION .. 
and its solution can be find out by the method used to find a potential function for a conservative vector field .. 
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..