How does the composition of a cover letter vary according to the situation?

The composition of a cover letter depends on whether or not the company has asked for applications.Two different hiring situations call for two different kinds of application letters.One must write a solicited letter when one knows that the company is hiring. Sometimes,however,the advertised positions may not be what one wants,or one may want to work for an organization that has not announced that it has openings in one's area.Then the situation calls for an unsolicited or prospecting letter.A prospecting letter will help one tap into the hidden job market.