Academic IELTS is an internationally-recognised test which is used as an entry requirement for studying at university in the UK. It is also often needed for visa applications and for people going into specific fields of study such as medicine. When you take an IELTS test, your result will be a in the form of a band scored from 1 to 9. When you contact a university, they will tell you the band you need to achieve to enroll on their courses.
IELTS is for overseas students wanting to go into higher education, professionals in particular fields (such as medicine) wanting to practise in the UK, and also for some people who need it for their visa application.