Psalm 96 picks up with a new song being sung which denotes a change. The old song was Moses in Exodus 15 when the people were saved from Egypt. 

This new song is sung by all the world. This is because a new salvation has occurred. A salvation for all people at the reign of God as King over the nations and as judge. Even the creation signals this change as Paul notes in Romans 8 that when the creation is restored it too will sing for joy. 

This Psalm also has its context in 1 Chronicles 16 when David brings in the ark to Jerusalem. 

Behind this Psalm stands the incarnation of Jesus Christ and his victory over death and sin and his enthronement as King and Judge.