This is a great question. Let me answer this question in a few bullet points.
1. The Old Testament is also called the Hebrew Bible. This fact gives us an insight. It is originally the writing of the Jews. This is their canonical scriptures.
2. The Hebrew Bible is broken into four parts: Pentateuch, Poets, Prophets, and Historical books.
3. The New Testament is the scriptures of Christians. However, they call it the New Testament, because Christians believe that the Old Testament is completed in the New Testament. In other words, Christians claim that the words and the prophesies of the Hebrew Bible find their fulfillment in Christ, the messiah. So, Jesus is the final prophet, priest, and king. He is also the final sacrifice. In short, he is God in the flesh to save people from their sins.
4. In light of point 3, the New Testament for Christians comprises of the work of Christ. For Jews, there is only one testament - the Hebrew Bible.