A cover letter is usually a business letter detailing the contents of a package (such as the application package) and the purpose of sending it. In this case it is your opportunity to tell us who you are, what you are interested in doing as a linguist, why you chose us to work with, and why we should admit you.
The most informative letters demonstrate familiarity and past experience in the field of linguistics, linguistic disciplines or language areas you are interested in, and especially for us who among our faculty offers the kind of advising you would benefit from most. At the PhD level, and to a lesser extent the MA level, we expect students to be able to articulate issues they are interested in pursuing as research areas, and the models and methods they may use to explore them. We do not regard anything you say in this regard as a promise--you are free (even expected) to find other interests, more interesting questions, etc. during your studies with us. Primarily we need to know you understand what we do and how we do it, and why you have chosen to study linguistics at the University of Manitoba.