It is an English Language proficiency test which is required to be taken by non-native English Speaking students for either immigration purposes or for studying abroad. It is more popular than TOEFL tests.
You cannot be accepted in the immigration process without IELTS
You are not allowed to study abroad without the need of either IELTS or TOEFL
It is a must to study in Australia, England, Canada and New Zealand.
You can repeat the test as often as you like.
It evaluates your English Language ability (reading, writing, listening and speaking).