In simple terms, we can say that the method declaration part is in one partial class and the definition part in another partial class or may contain declaration and definition in the same partial class.

for example:

Car.cs

public partial class Car
{  
    partial void CalcPrice(decimal price);  
}

Car2.cs

public partial class Car 
{ 
    partial void CalcPrice(decimal price)
    {
       price = price * 5000;
       Console.WriteLine("Car price is : {0}", price);
    }
}