Task 1 tests your ability to analyse data objectively without giving an opinion, whereas Task 2 usually requires a subjective piece of writing on a fairly general topic. In addition, it is worth noting that the exam is not testing knowledge of English language, but rather competence in using English. In other words, it is not testing memory. Awareness of this might help reduce some of the problems that many candidates have in the IELTS exam.
In the exam, the minimum word limit for Task 1 is 150 words and you need to spend about 20 minutes on this part of the test. Task 2 must be at least 250 words, on which you need to spend about 40 minutes. In both Tasks, there is no upper word limit.
Many candidates frequently exceed the minimum amounts by a very wide margin, which creates several problems. Rather than concentrating on producing a good essay, candidates write beyond what is necessary, thinking that there are extra marks for writing more. This is usually not the case.
It is very important that you try to keep the word limits, and perhaps write just a little more. You could write between 150 and 180 words for Task 1 and 250 and 300 for Task 2. If you write too few words, you will lose marks. While practising for the IELTS exam, count the number of words you write per line and then work out how many lines you need to reach the 150/250 word limit. It may surprise you how little you have to write! You could draw a line to mark the word limits when you are writing your homework. This will help train you to keep to the limits and help you to focus on where you are going and what you are aiming for.
One important reason for writing just a little more than the word limit is to give yourself enough time to check what you have written. During the actual exam, you should spend 6-7 minutes analysing the question, about 30 minutes writing your essay, and 3-4 minutes checking your essay for mistakes.
Note that the value of the marks given to each Task is reflected in the time. There are twice as many marks for Task 2 as for Task 1. The marks are combined to produce one Band Score from 1 to 9 for the whole test. Note also that if you write less than 150 words for Task 1 and less than 250 for Task 2, you will lose marks.