When we teach onsite (and online) technical, business, and scientific writing workshops, we always stress the importance of writers figuring out the image, or ethos, they wish to convey of themselves and/or their organization via the written document. Whether we're aware of it or not, readers make decisions about us and our compentence, professionalism, and intelligence based on our writing.
Think about a company like Nike (or any international company, for that matter). We can view a Nike ad for instance, and say that Nike is trying to sell us a product. But if we think critically about those Nike ads, we begin to understand that this isn't necessarily the case. If Nike were interested solely in selling us a product, they would show us the product, indicate what it's made of, and tell us the price. But they don't do that. Instead, they attempt to create an image (ethos) of the company itself--they support athletes of all kinds, are goal-oriented, etc. So, while the ultimate goal may be to sell us a product, the company is also interested in creating an image of itself via its advertising. In much the same way, we create an image of ourselves when we write.
As writers, it's important for us to "advertise" ourselves, just as any great company does; we "advertise" via our writing. Have you ever read a document and thought, "Wow, this person isn't very smart"? Most of us have, and that self image in writing is what's known as the writer's "ethos." The writer could be the smartest person the planet, but if that's not conveyed via the written document then, for all practical purposes, that person isn't smart.
How do we convey ethos? Ethos is conveyed in a document that's easy to read and understand; that clearly shows an accurate assessment of, and appropriate message for, the audience; and in a document that has a clear purpose that the reader can quickly and easily find and understand. Ethos is one of the most important elements of effective technical, business, and scientific writing. If a poorly written document presents us as unprofessional, unintelligent, and inarticulate, then we've clearly lost that reader and, unfortunately, "advertised" ourselves and/or our organization in the same way. Before writing, think carefully and write down how you wish to portray yourself and your organization--doing so can go a long way toward helping you create that positive ethos in your writing.