Prelims is the first stage of IAS examination and Mains is the second stage in which answers should be hand-written in a question-cum-answer booklet supplied by UPSC. Prelims cover subjects like Indian History, Indian and World Geography, Economics, Indian Polity, Science and Technology, Environment, and Current Affairs. And Paper 2 (CSAT) covers topics like Comprehension, Logical Reasoning, Mental Ability, Quantitative Ability etc. First prepare for these topics and analyse syllabus for mains along with it. If you are preparing for Prelims, keep focus on mains as well.