The International English Language Testing System (IELTS) is a usual entrance requirement by British, Australian, New Zealand and Canadian universities and for secondary, vocational and training programs.Now increasingly American universities have started accepting valid scores in this exam.
IELTS is designed to assess the language ability of candidates who need to study or work where English is used as the language of communication. Tests are administered at accredited Test Centers throughout the world - there are currently more than 300 Centers, in over 120 countries.
 
Academic and general training candidates

Candidates must select either the Academic or General Training Reading and Writing Modules depending on the stated requirement of their sponsor or receiving institution. The Academic Reading and Writing Modules assess whether a candidate is ready to study or train in the medium of English at an Undergraduate or Postgraduate level. The emphasis of General Training is on basic survival skills in a broad social and educational context. It is suitable for candidates who are going to English speaking countries to complete their Secondary Education, to undertake work experience or training programs not at degree level, or for immigration purposes.

Test Format
 
All candidates are tested in listening, reading, writing and speaking. All candidates take the same Listening and Speaking Modules. There is a choice of Reading and Writing Modules. The first three modules- Listening, Reading and Writing must be completed in one day. The Speaking may be taken, at the discretion of the test center, on the same day or up to two days later. Candidates in some test centers are required to take additional pretest sections of up to 20 minutes. Performance on these pretests does not affect a candidate's results in any way but pre-testing is an essential part of IELTS question paper production.

General Tips

Success in the IELTS exam requires a candidate to know the test format and the specific techniques for answering questions. Make sure you are fully equipped with this knowledge. ( Please see Resources on the Internet and Books ). Don't believe people who tell you that IELTS needs no preparation if your English is good. Even if it is, you still need to learn the right skills for the test. We  would suggest a period of two weeks as preparation time, though this would vary depending on your level of familiarity with English. The test fees are high and if you don't get the band score you need, you have to wait for three months before you can take the test again. Like in all other exams, practice is the key to doing well in the IELTS. Make sure you have plenty of it before you appear for the test.Answer all questions. There is no negative marking for incorrect answers.
Preparing for the test
Start by familiarizing yourself with the test format. After that you could start with Step Up to IELTS to build the basic skills that you will need to do the test. Lastly, go on to doing the 4 tests in Cambridge IELTS 3 under test conditions. This should get you ready to take on the IELTS. 

Source: http://www.aippg.com/ielts/