The Purpose of a Cover Letter
What is a cover letter?
The cover letter narrates who you are, summarizes other documentation that you have supplied (research statement, CV, teaching statement, etc) and asserts why you are a fit for the department. In your cover letter, identify and convey how you differentiate from many other applicants for the position by highlighting the unique value of your academic contributions that will add to the department. Include concrete examples, reference prior experiences and include clear evidence to support your claims.
How much do I research the department?
In your letter, you should demonstrate that you have done your research on the department, the university and other on-campus locations you'll be working at or collaborating with. Most importantly, you are representing yourself as a colleague and persuading the committee that your research/teaching expertise significantly contributes to the department.
Research can include:
Who is in the department and what is their research?
Is there anyone in the department you can collaborate with?
Does the department have any kind of institutional affiliations or labs, off- or on-campus?
What courses are in high demand? And what courses could you offer to meet a certain demand?
What are the goals/values of the University, and how does the department reflect those?
Has the University/department received funding for a particular form of research that fits your research?