The difference between the Preliminary Examination and the Main Examination in UPSC lies in their structure and purpose. The Prelims consist of objective-type questions designed to screen candidates based on general knowledge and awareness of current affairs. In contrast, the Mains comprises descriptive-type questions that assess candidates' analytical abilities, depth of understanding, and writing skills across various subjects. The Prelims function as a qualifying round, while the Mains evaluate the candidate's suitability for the civil services, ultimately determining who advances to the subsequent interview stage.