An essay is a well-researched and logically structured answer to a particular question, or questions, usually presented as an argument. It is a point of view formulated by critically assessing the information or ideas relevant to the essay topic.
It is presented in the form of a series of main points, which support your direct answer to the question. Each of these points is addressed in a separate paragraph and is supported with evidence, explanation and/or examples. The argument presented in an essay should be supported by referencing authorities in the relevant field.
The argument should also form a cohesive whole: this means the paragraphs need to be logically ordered and connections made between the points presented in those paragraphs. Essays are used as an assessment tool to evaluate your ability to research a topic and construct an argument, as well as your understanding of subject content.
This does not mean that essays are a 'regurgitation' of everything your lecturer has said throughout the course. Essays are your opportunity to explore in greater depth aspects of the course - theories, issues, texts, etc. - and in some cases relate these aspects to a particular context.
It is your opportunity to articulate your ideas, but in a certain way: using formal academic style. In any type of writing or presentation you need to consider the institutional context (the university), and your audience (who will be reading your essay). These elements influence the style and tone of your writing.
In most instances your writing should be formal and typically objective. This means everyday language and slang as well as unsubstantiated opinion are unsuitable in the context of an academic essay. Furthermore, students write essays for their tutors and lecturers: in other words, as a student you are in the uncomfortable position of writing about a topic for someone who most likely knows more about it than you do! You are writing for someone who is familiar with the content, as well as the conventions and practices of the discipline, and in your own writing it is expected that you adapt your writing to suit this context.