The IELTS (International English Language Testing System) is jointly owned and run by three organizations: the British Council, IDP: IELTS Australia and Cambridge English Language Assessment. It is accepted not only by universities and corporations but by the immigration departments of several countries.
There are two basic forms of the test. Both use the same Listening and Speaking sections, but they have different versions of the Reading and Writing Tests. The Academic Test, like the TOEFL, is for people who want to study at universities or colleges in English-speaking countries. The General Training Test is for employment, immigration, or high school purposes.
One major difference between the IELTS and the TOEFL or TOEIC is that the IELTS speaking test is face to face with an examiner. It may be given on a different day than the other tests. (In the American tests the test-taker speaks into a microphone and his or her responses are recorded and later graded by a panel rather than a single examiner.)
The IELTS site (https://www.ielts.org/) has a lot of practice material to help you prepare. There is more test information in a short summary pdf downloadable from Cambridge, or a detailed study guide to the IELTS provided by Magoosh.