A cover letter is sent to a potential employer along with your résumé. A cover letter is important because it will tell the recipient for whom the résumé is intended and it can elaborate on your knowledge of the company and your desire to work for it. Most employers will want you to submit an organized cover letter to support your employment application and resume submission.
Each cover letter must be produced for a specific employer. You can recycle some of the text for cover letters within the same industry, but each letter should contain a reference to a specific detail about the employer or company. You should research important business or technological trends within your industry, significant recent transactions, hiring trends and popular terms, phrases and subject matter.
Learn all you can about the history of any company to which you apply. Identify its major clients, review any important recent transactions, and study its hiring trends and employment needs. The more you know about an industry and a particular company, the more effective your cover letter will be.