What is the difference of the old testament and the New Testaments?

The Old Testament prophesys of the Christ:

"...all things must be fulfilled, which were written in the Law of Moses, and in the Prophets, and in the Psalms, concerning Me." (Luke 24:44)

The New Testament illuminates the spiritual aspects of the Old, records the beginning of the fulfillment of "all those things" written in the Old... and details the ongoing fulfillment of the coming "harvest" and completion of God's plan.

"...the harvest is the end of the world [age]; and the reapers are the angels." (Matt.13:39)

"...God shall wipe away all tears from their eyes; and there shall be no more death... Behold, I make all things new... It is done..." (Rev.21:4-6).

The New Testament COMPLETES what the Old began.

A great spiritual darkness exists on earth, that will be removed when Jesus Christ, the Light, returns to fulfill the last of "all the things" that must be fulfilled in God's plan.

These things are foretold in the Old... and are being fulfilled now as the writings of the New Testament unfold and are revealed at the end.

"Bind up the Testimony and seal up [complete] the law among My disciples.

"I will wait for the LORD, who is hiding His face from the House of Jacob. I will put my trust in Him... When men tell you to consult mediums and spiritists, who whisper and mutter, should not a people inquire of their God? Why consult the dead on behalf of the living?

"To the law and to the testimony! If they do not speak according to this Word, they have no Light of dawn [or, they have no Light {the Spirit of Christ} in them].

"Distressed and hungry, they will roam through the land; when they are famished, they will become enraged and, looking upward, will curse their King and their God. Then they will look toward the earth and see only distress and darkness and fearful gloom, and they will be thrust into darkness." (Isa.8:16-22 NIV)

The prophecy above is talking about the returning Jesus Christ [the Light of the New Testament]. It's talking about the soon-to-be-fulfilled Great Tribulation... the "darkness" into which the world will be thrust. It's talking about a distressed and hungry world population enraged at seeing the Light of Christ cutting through the gloom and clouds that will cover the earth at His return.

"...you have taken your great Power and have begun to reign. The nations were angry; and your wrath has come. The time for Judging the dead, and for rewarding your servants the Prophets and your saints and those who reverence your name, both small and great -- and for destroying those who destroy the earth." (Rev.11:17-18 NIV)

"...the sun will be darkened, and the moon will not give its light; the stars will fall from the sky, and the heavenly bodies will be shaken... the sign of the Son of Man will appear in the sky, and all the nations of the earth will mourn. They will see the Son of Man coming on the clouds of the sky, with Power and Great Glory." (Matt.24:29-30 NIV)

The Old Testament was recorded for the end time generations to see what's coming in the plan of God. The New Testament records for them the fulfillment of the "early spring harvest" prophecies [the Passover - Jesus' sacrifice; Pentecost - the indwelling of the Holy Spirit], and testifies to the ongoing fulfillment of the coming "fall harvest"... Judgment Day with the return to earth of Jesus Christ. In this... the two testaments are different.

Taken together... they compliment each other... and in the inspired words of Isaiah... the disciples "completed" the Word of God as a whole for those in the end time, who have the Light in them, to see what's coming on the earth.

Another Answer

Very basically, the Old Testament is the history of Israel as a nation which observed the Sabbath [ "memorial of Creation"] and looks back to Creation, while the New Testament is the history of a Man, Jesus Christ, and His resurrection and looks forward to His Second Coming.