There are two parts to the IELTS writing paper.
In part one you have to look at a graph, or a diagram, or a table, or a picture of a process and present the information in your own words. You are told to write at least 150 words and to spend about 20 minutes on this question. The exam is testing your ability to describe data or a process, or explain how something works.
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In part two you are asked to write a longer academic essay on a topic. You need to present an argument and discuss ideas. The exam is testing your ability to present and weigh up arguments in an academic style.