A cover letter is a letter to an employer that says which position you're applying for and includes an overview of your skills. You should always include a cover letter when you mail or E-mail your resume to an employer, even if they don't ask for one. Cover letters are actually a great way to set yourself apart from the rest if you write it correctly.
Cover letters allow you to actually get your personalized message across to the hiring manager or possible interviewer. The key is to be brief and direct...just like a resume...the person on the other end will want to quickly read your cover letter...so make sure you don't write a short story. Use the cover letter to peak the interest of the reader. The purpose here is to get the reader interested so that he/she will want to examine your resume.
Everyone who sends out a resume does need a Cover Letter! Even if the cover letter never "came up" in conversation or wasn't mentioned in an advertisement, it's expected that you will write one.
Your cover letter can be A Hard Copy or in Email format. The main difference between e-mail and hard copy cover letter is format: your signature block (address, etc.) goes below your name in e-mail, while it goes at the top of the page on hard copy. Of course you won't have a handwritten signature on e-mail, but don't forget this on hard copy.
Here are some of the things that one should never forget to include in the Cover Letter: