An essay is, generally, a piece of writing that gives the author's own argument — but the definition is vague, overlapping with those of a paper, an article, a pamphlet, and a short story. Essays have traditionally been sub-classified as formal and informal.
A synthesis essay uses a fairly standard format that consists of an introduction, a body and a conclusion. It is a format common to many essays, and one that should be familiar to most students. The introduction generally consists of one paragraph. One method of learning how to write introductions for synthesis essays is to review a synthesis.