Fundamentally, an essay is an argument – it is an extended written argument, which brings evidence and reasoning to bear in an attempt to persuade the reader. There are, of course, different sorts of essays; some essay titles encourage you to take a stance, others seem to be asking for more of a balanced discussion. But even essays titles that end with the instruction 'Discuss' are asking for an argument rather than for a book report or a shopping list. It is never a good idea just to produce a meandering review of your reading, nor a shopping list of different facts and different opinions vaguely related to the theme of the essay. You do, of course, need to get a certain amount of information across – you cannot persuade your reader without appraising her of the relevant information along the way – and you also should show that you have understood the reading you have done. Your first priority, however, and the task that should motivate everything that you write in the essay, and the way that you structure it, is the job of arguing your case.
You must always show that you are aware of the evidence and arguments deployed by those you are arguing against in your essay – and you should give a fair and accurate account of those views, rather than setting up a very weak or exaggerated version of their views that is very easy to knock down – a 'straw man'. So you must understand and give an accurate account of views on different sides of the argument, but you do not have to defend all sides of the argument, nor give a totally even-handed account of each side.
In short, argumentative essays are more engaging and interesting to read than directionless discursive ones, but strong argumentation should always go along with a clear understanding of the views being argued both for and against.