There are two types of cover letters: those that draw the reader in and entice him or her to read your resume, and those that eliminate you from the running before your resume gets read.
The cover letter serves as the introduction to your resume. No resume should ever be sent out without one.
While your resume can remain the same for every company to which you apply, the cover letter should be created separately and individually for each position for which you express interest.
Your career objective should be included in the cover letter - not the resume. By including it in the cover letter (which has to be written separately anyway), you can tailor the objective to the advertisement to which you are responding or to the tip you received through your professional network.
Studies indicate that your cover letter has a life expectancy of about eight seconds, so it must be brief, yet informative.
The letter should have three recognizable parts: the opening, which explains why you are writing; the main body, which lists two or three of your major accomplishments and gives the reader good reason to want to interview you; and the closing, which expresses a desire for future communication.
Be sure your letter draws a connection between the needs of the current job opening and skills you can bring to the job.