It is designed to evaluate a persons' understanding of current affairs and tests their ability in short duration of time. The preliminary paper is a compulsory and qualifying paper. Aspirant must clear the prelims exam to qualify to the next stage of the exam called Mains Exam. The prelims exam has two papers General Studies Paper I and General Studies (CSAT) Paper II (qualifying in nature) carrying a maximum of 400 marks.
The mains syllabus for optional papers is generally of the honours degree level that is between the bachelors degree and the masters degree. It contains 9 papers, where there are two qualifying papers and seven papers counted for ranking and syllabus is framed according to the subjects with total of 1750 marks.
The UPSC Interview/personality test has no prescribed syllabus. The candidates will be interviewed by a Board of competent and unbiased members and will be questioned on matters of general interest and the candidate will be evaluated not only based on their intellectual qualities but also on social traits, interest in current affairs, mental alertness, critical power of adaptation, rational exposition, balance of judgment, variety and depth of interest, ability for social interconnection and leadership, logical and moral uprightness
Here is a list of subjects that can be taken as optional paper in the Civil Services (Mains) examination. The list is as per the UPSC prescribed subjects. Once you click on the listed links you will find all resources regarding that IAS exam paper and the syllabus. Knowing the IAS Syllabus thoroughly is one of the most important sections of making through the IAS exam conducted by UPSC.
UPSC optional subjects List for Civil Service (Mains) Exam
UPSC Optional subjects for Indian Languages