What makes expository writing distinct from other forms of writing?For an English class.

Essays are generally categorized into
four groups: narrative, expository, descriptive, and
persuasive.  These categories distinguish between the different
purposes of a piece of a writing.  Narrative writing
tells a story.  It has a clear beginning, middle, and end, and most
often moves chronologically.  Descriptive writing is exactly that:
descriptive.  Often scientific in nature, descriptive writing gives
details about the subject in order to provide a full explanation.  Persuasive writing
attempts to argue a point and persuade an audience to
agree.

Compared to each of the above forms,
expository writing has the most broad definition.  Its purpose is to explain, inform,
describe or define a subject for the reader.  In this way, expository writing can take
on many of the characteristics of the other three modes (from above).  Expository
writing can incorporate stories and figurative language, making it somewhat narrative. 
It can be very detailed in description and even have minor elements of persuasion (as it
is often difficult for an author to be completely emotionless or without personal
opinion).  On the whole, the expository writing mode is meant to
inform.

Common essay assignments which
fall under the category of expository essay are compare-contrast, how-to, and extended
definition essays.