Hi

Both jee mains and jee advanced are entrance examination conducted for providing admission in undergraduate  engineering courses

Here's a brief of both JEE mains and JEE advanced

JEE MAINS

*Through JEE mains admission in NITs ,IIITs, gfti ,CFTIs etc is provided.

* it is conducted by NTA

*It is conducted 4 times a year

*Eligibility criteria says that to be eligible  for JEE mains 2021 for BE or Btech you should be either appearing for 12th board exam or equivalent  in 2021 or should have passed it in 2020 or 2019 with Physics,  mathematics and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

* You can attempt JEE Mains for three consecutive years starting from the year you passed your 12th

For detailed information about JEE mains go through

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

JEE ADVANCED

* JEE mains is conducted to provide admission in IITs in undergraduate engineering courses. Some institutes other than IITs also accept jee advanced like Indian Institute of Science (IISc), Bangalore, Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram etc

* it is conducted by IITs themselves

* it is conducted only once in a year

* to be eligible you must qualify JEE mains first and also  need at least  75% marks in aggregate in class 12 if you belong to general or obc category or at least  65% marks if belongs to SC/ST or  must be in the top 20 percentile in  class 12 examination.

* one has a maximum of two attempts for JEE advanced in 2 consecutive years

You can get detailed information about JEE advanced at

https://engineering.careers360.com/exams/jee-advanced

Thank you