What is the difference between "introduction" and "motivation" sections
Introduction: WHAT
The introduction of a scientific publication is where you explain the background of your research. What are you building on? What does the reader need to know to properly understand your presented work? What is your hypothesis (if this sort of thing is relevant to your field)?
Motivation: WHY
The motivation section explains the importance behind your research. Why should the reader care? Why is your research the most groundbreaking piece of scientific knowledge since sliced bread (or CRISPR/Cas9 genome editing)?
Some people do combine the introduction and the motivation, it really depends on the research and the researcher. There is generally a fair amount of overlap between introducing related work, and then saying that your work goes above and beyond this previous research. Remember that these are just two of the big questions you need to answer with your paper. Others include:
Methods: HOW
Results: WHAT HAPPENED
Discussion: SO?