The word Essay is defined as "a literary composition on any subject." Properly speaking, it is a written composition giving expression to one's own personal ideas or opinions on same topic. It should be good to remember that the word essay means, literally, an attempt. The essays you write at school are trial exercises or "attempts" to express your thoughts in good English.
Now, what are the points we should keep in mind for writing a good essay?
1) Unity-An essay must be a unity, developing one theme with a definite purpose. The subject must be clearly defined in the mind and kept in view throughout. Nothing that is not relevant to it should be admitted to the essay.
2) Order-The essay should follow a certain ordered line of thought and come to a definite conclusion. It should not consist of haphazard reflections put down anyhow. There should be not only unity of subject but also unity of treatment. Hence there is the necessity for thinking out a line of thought before beginning to write.
3) Style-the style of an essay should be dignified and literary. Slang, colloquial terms needed to be avoided in an essay. At the same time it is a mistake to make an essay an attempt of any fancy writing. The language and sentence construction should be simple, direct and natural.
The secret of clear writing is definitely clear thinking.
4) The Personal Touch-An essay should reveal the personal feelings and opinions of the writer. It should have its individuality in it. Strictly speaking, an essay is a written composition giving expression to one's personal ideas or opinions on a subject. If this personal touch is lost, the essay would appear to be colorless and devoid of any individuality.
To sum up, an essay must be a unity, treating in an orderly manner of one subject; it should be concisely written and not too long, and the style should be simple, direct and clear; and it should have individuality, or reflect the personal touch of the writer.
Three features necessary in a good essay are-1) suitable subject matter,2) proper arrangement and 3) adequate power of expression. Point 3 is applicable to most forms of writing like writing short stories, reviews, and reports.