An expository essay is like any formal essay when stressing on the format. But, the style of an expository essay is different from the other essays by the fact that an expository essay is completely fact-filled - there is no place for one's own view on the topic or any thesis. From the start till the end, every statement regarding the topic will be based on facts - unquestionable facts.
The format although remains the same as in introduction, body and conclusion; yet the matter will differ from the other generally written essay. In the introduction, the author will have to write about the topic with a plain path to the factual discussion. Although writing an introduction without having to say something on your own may be difficult, but the expository essays never allow one's opinion about the topic anywhere in the essay.
There are topics like 'risks of changing access passwords frequently' or 'effect of climate change on living organisms' which can be elaborated with plenty of facts building a complete essay. The body should contain all the facts one needs to mention regarding the topic in a flowing format for proper understanding of the reader.
The conclusion of an expository should be speaking nothing new about the topic and should stress completely on summarizing the already mentioned facts. There should be no personal inferences bringing the study to end; rather, it should be a plain and simple end with acting like a précis to the whole fact based matter.