For the GRE General Test, you must write an issue and an argument essay. The two essays are quite different.
The Issue Writing Task: You are presented with a topic -- often a controversial one. You are asked to present your perspective on the issue, using examples to support your claim. You have 30 minutes to plan and compose a response.
The Argument Writing Task: You are given a single topic, which often includes a snippet of an argument or a snippet of an editorial. In writing the argument essay, your goal is not to agree or disagree, but instead you must critically analyze how well reasoned you find the provided statement. You have 30 minutes to plan and compose the argument essay.
To achieve a top score, you'll need to demonstrate that you can write clearly and effectively. To that extent you'll need to accomplish the following:
Clearly and effectively articulate complex ideas
Examine claims and discuss accompanying evidence
Provide examples to help support your claims
Make sure your writing remains focused
Your score on the essay section of the GRE exam is used by graduate schools to evaluate your writing and analytical skills. Two separate trained readers will read your essays. They will score each essay based on many factors, and the scores will be combined and averaged. Scores are given on a range from zero to six, in half-point increments. The ETS provides the criteria that are used to score the issue and argument essays. Although in the essay section grammar and spelling are important, it is much more important that you concentrate on writing a coherent, complete, and persuasive essay.