The difference between the Preliminary and Mains stages of the UPSC examination lies in the format and purpose. The Preliminary Examination consists of two objective-type papers that assess general knowledge and current affairs, serving as a screening tool. In contrast, the Mains Examination consists of written descriptive papers that evaluate candidates' in-depth understanding and analytical abilities across various subjects. The Mains requires candidates to articulate their thoughts clearly and demonstrate critical reasoning, reflecting the higher complexity and expectations of the Civil Services selection process.