A cover letter is a brief letter that could be part of a job application if requested by the employer, or that you could include with your resume when contacting employers. It explains why you are applying for the job, highlights the key points from your resume by giving examples, and demonstrates that you have read the job advertisement or have an understanding of the role you would like to be considered for.
A cover letter should be no more than a page, and shouldn't include information that isn't supported by your resume. Do not introduce anything new, as that might confuse the employer.
Do not send the same cover letter to every application. Make every cover letter specific to the job you are applying for. If you are responding to a job ad, you should address the selection criteria or requirements in your cover letter.
Is a cover letter needed?
Your cover letter is designed to grab the interest of the reader and encourage them to find out more. If you are contacting an employer directly you can send them a cover letter with your resume, unless they ask you not to. If responding to a job ad, you should include a cover letter if it is asked for or not mentioned, but do not include one if the ad specifically says not to.
Can I use a cover email instead?
If you are emailing your resume to an employer, you will need to include some text in the email. You can either use this as your cover letter, or write a short and polite email stating that your cover letter and resume are attached.
If the employer specifically asks for a cover letter in a particular format, you should always do what is asked.