The main purpose of a cover letter is to give a personal introduction from you to the company you are trying to get hired at. The cover will ultimately give the hiring manager a reason to check out your resume and want to learn more about you and how you can help the company.
Your cover letter needs to do the following things:
1) Let the hiring manager know what position you are applying for.
2) Highlight your KEY points and qualifications.
3) Address the needs of the employer and make them want to find out more about you and what you have to offer.
4) Ask the hiring manager to contact you and list how and when you will follow up with them as well.
Your cover letter should just be an introduction, not your entire life story. You do not want to cover everything that you have done in your past, you just want to make sure that the key points are covered and covered well.
Make sure that you write a new and unique letter for every position that you are applying for, cover the four main points and make the reader want to find out more. Then, follow up with an equally amazing resume.
The cover letter is intended to be a hook that gets the reader interested and your resume should make the sale! Just highlight the important skills and your experience as it pertains to this specific job opening and target towards one specific employer and you will be on your way to an interview!