You should always send a resume cover letter with your resume. Remember, your employer sees your cover letter first (in email) and then only your resume. If your cover letter is not good enough, employer may loose interest in your resume. So here are some of the tips you should remember while writing your resume cover letter:
Your cover letter should be so accurately targeted at the job requirement, that the employer should be eager to read your resume. The cover letter is a perfect marketing tool and the foundation of any resume.
For various types of cover letter, explore Cover Letters