Introduction:
The Joint Entrance Examination (JEE) is a highly competitive entrance exam conducted in India for admission into various undergraduate engineering programs. It is divided into two parts: JEE Main and JEE Advanced. While both exams are essential for engineering aspirants, they differ in various aspects.

Eligibility:
- JEE Main: All candidates who have passed their 10+2 or equivalent examination are eligible to appear for JEE Main.
- JEE Advanced: Only the top 2,50,000 rank holders of JEE Main are eligible to appear for JEE Advanced.

Exam Pattern:
- JEE Main: The exam consists of two papers - Paper 1 for B.E./B.Tech programs and Paper 2 for B.Arch/B.Planning programs. Both papers have multiple-choice questions (MCQs) and are conducted in online or offline mode.
- JEE Advanced: The exam consists of two papers - Paper 1 and Paper 2. Both papers have objective-type questions as well as numerical answer type questions. The exam is conducted in online mode only.

Difficulty Level:
- JEE Main: The difficulty level of JEE Main is relatively lower compared to JEE Advanced. The questions are designed to test the basic understanding and application of concepts.
- JEE Advanced: JEE Advanced is known for its higher difficulty level. The questions are more complex and require a deeper understanding of concepts, logical reasoning, and analytical skills.

Topics Covered:
- JEE Main: The syllabus of JEE Main is based on the topics taught in class 11th and 12th in Physics, Chemistry, and Mathematics.
- JEE Advanced: The syllabus of JEE Advanced is more extensive and includes additional topics beyond the JEE Main syllabus. It covers advanced concepts and a higher level of problem-solving skills.

Admission into IITs:
- JEE Main: The exam is primarily used for admission into National Institutes of Technology (NITs), Indian Institutes of Information Technology (IIITs), and other centrally funded technical institutions.
- JEE Advanced: The top rankers of JEE Advanced are eligible for admission into the prestigious Indian Institutes of Technology (IITs) and other premier engineering colleges in India.

Conclusion:
In conclusion, JEE Main and JEE Advanced are two different stages of the Joint Entrance Examination. JEE Main serves as a screening test, while JEE Advanced is the final step for admission into the top engineering colleges in India. The exams differ in terms of eligibility, exam pattern, difficulty level, topics covered, and the institutes they lead to. Aspiring engineering students need to prepare accordingly to excel in both exams.