The difference between the Prelims and Mains in the context of UPSC lies in their structure and purpose. The Prelims consist of two objective-type papers aimed at screening candidates, assessing general studies knowledge and aptitude through multiple-choice questions. In contrast, the Mains examination comprises nine descriptive papers where candidates must provide in-depth written responses, testing their analytical skills and understanding of subjects. The Mains evaluate a candidate's comprehensive grasp of the syllabus, critical thinking, and written expression. Success in Prelims is necessary for advancing to the Mains, which has a more substantial impact on overall selection.