What is joy?
a feeling of great pleasure and happiness.
What is the joy of the Lord and how does it differ?
The joy that comes from God differs from the joy that comes from mental stimulation. Your spirit inside can rejoice also. The joy that comes from the spirit is God's joy and this joy cannot be obtained any other way.
Psa 16:11 Thou wilt shew me the path of life: in thy presence is fulness of joy; at thy right hand there are pleasures for evermore.
David says, Lord my life is filled with activities. I trust you will shew me the way I should respond to things in life. How I to do things the right way. The outcome from my obedience and the reward of your presence brings me great joy. This my Lord is the fulness of joy and at thy side is pleasure that remains in my experience and testimonies forever.
Isa 12:3 Therefore with joy shall ye draw water out of the wells of salvation.
The type of joy that comes from God is the realization that everything is an opportunity to be close to God, to have him walk with you and teach you to grow spiritually and from that is the fulness of joy from the Lord.