The writing task 2, which is often called as Essay writing, is marked using scores from 1 to 9 on four criteria, as follows:
Task Response: This means how well one answered the question. The examiners look at whether a candidate addressed all parts of the topic and answered each question precisely. In addition, they also check whether a test taker writes equal amount of writing for each part of the question.
Coherence and Cohesion: This means how well a person connects paragraph and sentences in each paragraph. In fact, Coherence means logical connection between paragraphs and Cohesion means logical flow of sentences using appropriate connective devices.
Lexical Resource / Vocabulary: This means how well a candidate use different words and not to repeat the same vocabulary again and again. Use a wide range of vocabulary in a precise manner to score higher.
Grammatical Range and Accuracy: This means a candidate has to use a wide range of structures with accuracy. An examiner will not only look for the tenses, but also check whether you organise your sentences well.
For detailed information, click here to download the public version of IELTS Task 2 Writing band descriptors.