Generally you have had your resume given to the potential employer by a friend, colleague, or forwarded by another department, etc. Now you need to take the opportunity to relate yourself more specifically to the needs of the potential employer.

Ask questions of whomever you can, research if possible what is going on in the company. Piece together a proposal for what you could do for them that is based on some knowledge of their needs.

If you left a resume in person for a potential interviewer, you should have told the person on the receiving end what you are interested in (you were the cover letter). If you did not have this opportunity, take the time to find out what is going on inside this employer before you introduce yourself again in a letter.