What is the difference between a population and a sample?

What is the difference between a population and a sample? How large do you think your population and sample will need to be for your dissertation research? My dissertation research topic is General Education Teachers and the Influence of Tay-Sachs Disease on the Academic Performance and Temperament and Emotion of Childrens Performance and Achievement. Explain your reasoning. What factors other than size will you consider in relation to sampling? Why? What types of descriptive statistics will you consider for summarizing the sample? Why? Please ensure that the sources are peer-reviewed