A query letter is written to pitch an idea to an editor. The letter typically consists of an introduction to the proposed piece, the projected word count and completion date. Many people advise opening the letter in the tone and style you would use to write the article--like a teaser. The last part of the letter gives a brief overview of your experience. The final part of the letter functions as a brief resume citing your previously published work and/or relevant experience which qualifies you to write the article.
For a writer, a query letter is a job application. Like any job search professionalism and presentation are key. Consistency and persistence also play a big part. A recent blog, http://queryaday.blogspot.com, takes this philosophy to heart as its writer endeavors to place a query daily in her effort to find freelance work. Click on the link in this post's title and check it out.