International English Language Testing System or known as IELTS is an English proficiency test for non-native speakers wishing to study or work in English speaking countries. It was established in 1989 and managed by three major expert bodies; Cambridge English Language Assesment, the British Council and IDP Education.
Alongside with TOEFL and TOEIC, IELTS is one of the most reputable English language tests in the world. It is recognized by most universities in the UK, Ireland, Australia, Canada, New Zealand, South Africa, over 3,000 institutions in the US, and numerous certified organizations worldwide. IELTS is also part of immigration prerequisite if you want to migrate to Australia, New Zealand, and Canada.
IELTS has no minimum score for someone to pass the test and is valid only for two years. A "band 0" score is issued to a candidate who did not attempt the test whereas "band 9" is given to an expert user. Every institution has a different requirement.