Solar thermal collectors play a vital role in harnessing renewable energy for heating and power applications. Their performance depends on design parameters such as collector type, absorber plate material, glazing, insulation, tilt angle, and working fluid. This study presents a detailed review and analysis of design factors and performance evaluation methods for solar thermal collectors, emphasizing flat-plate and parabolic trough systems. Using secondary data and reported experimental studies, the paper identifies efficiency trends, heat loss mechanisms, and optimization strategies. The results highlight that proper selection of absorber coatings, insulation thickness, and flow arrangements can improve efficiency by 10–15%. In addition, advanced nanofluids and hybrid designs further enhance thermal output. The paper concludes that solar thermal collectors, if optimized, are sustainable and scalable solutions for meeting growing energy demands, especially in emerging economies.