The Tanakh and Old Testament contain the same content but are organized differently. The Tanakh counts 24 books while the Christian Old Testament counts 39 books.
Differences: Samuel, Kings, Chronicles, and Ezra-Nehemiah are each one book in the Tanakh but split in two in the OT.
The 12 Minor Prophets (Trei Asar) are counted as one book in the Tanakh.
Hover over any book to see its correspondence.