A letter of application (also called a cover letter) is designed to introduce the resume and stimulate the employer onto reading it.
An unsolicited letter of application,which is mailed to a company for which you would like to work although the company has not advertised that there are any openings.
An application letter in response to an advertisement in newpaper or periodical for a particular position.
In both cases, an application letter must be sent with an accompanying resume.