Cover letters are key components of a good, solid job search. While the resume serves the main purpose of getting the interview, the cover letter holds its hand while it's doing its job. A good cover letter is more than a rehash of the same information that appears on the resume. It provides key information that highlights the job seeker's candidacy and brings forth tidbits of information that would be helpful to the hiring manager.
Now days, since resumes are mostly emailed, cover letters are generally copied/pasted into the body of the email to which the resume is attached. If a resume package is to be hand delivered or mailed snail mail, the cover letter is still printed out on matching paper as has been customary in the past. When sending paper copies, make sure to SIGN the cover letter (many people overlook this). When sending via email, make sure you have a good signature line that you insert into all your emails.
Using an automatic signature is a good idea for all your emails. A good signature provides your contact information in case your resume gets corrupted in transmission and the reader has to email you back. Your email signature should include your name, your email address, a telephone number (cell or home), and possibly a brand such as IT Program Manager or Chief Information Officer - just something that puts your goal into the mind of the reader automatically.
You might think that executives who are used to communicating heavily would pay close attention to making sure their contact information is easily obtainable, but we are continually surprised how many highly paid professionals forget this small detail. If someone's email address reads jb123@xyz.com and he only signs it JB, I don't know who the person is. If the resume can't be opened for some reason, the recipient won't go to extra trouble to try to contact the sender; hiring managers just don't have time.
So think ahead to your communication package and make sure you have a strong cover letter but also that your email communications are clear and your contact information appears there, too.