Discussion
The discussion section of a scientific paper should interpret the results of your research. First, briefly remind your reader of your research question and principal findings by briefly restating these points. Then explain the results themselves. Discuss how they fit (or do not fit) your hypothesis, and whether they are consistent with the results of similar research projects. Did you encounter anything surprising or idiosyncratic? If so, why is it significant? What might have caused it? Build on the research question you posed in the introduction, and the context you established in the literature review. Make a case for the meaning and significance of your findings, and support your case by connecting it to related research.
Acknowledge other possible interpretations of your results, and admit your project's limitations. Your argument will be more convincing if you can anticipate your reader's potential objections to your claims and address them directly in the discussion section. For example, generalizability (or how applicable a study's results are to a more general population) is more limited with a smaller or less homogeneous sample. If your research sample is small or limited, be sure to acknowledge those limitations and address how they might have affected the results.
If your interpretation has broader implications, you can either suggest them in the discussion section or introduce them in a separate conclusion. You don't have to write a conclusion if your points fit neatly into the discussion section, but a conclusion is helpful if you want to make suggestions that stretch beyond the scope of your project.
Conclusion
The conclusion section is not strictly necessary in the social sciences, but it can be helpful to provide a succinct summary of your work. It is also a good place to make bold speculations about the implications of your project. You should discuss, somewhere in your paper, the significance of your research for future research, public policy, personal decision-making, or other spheres of influence. But think carefully about whether you could benefit from the distancing effect of putting these implications in a separate conclusion.
The conclusion should not repeat your discussion section. It should take one to three paragraphs to restate the research question, the main results, and the meaning of those results. The conclusion then reaches beyond the suggestions you made in the body of the paper to emphasize the importance of the results and their potential consequences.
Conclusion
Conclusions have been an important part of writing for centuries.