The letter of recommendation from a previous employer is critical for the prospective employment opportunities. The letter should confirm the credibility of the employee, and should aptly describe his skills and strong points. The content inside the letter is situation dependant and would differ according to the nature of employment. These letters are generally secret and the content is not even shown to the candidate. These letters can also be directed towards universities for application in graduate programs.
Normally your previous employer would not know the name of the prospective boss, so the letter begins with ‘to whom it may concern’. That would change in case of university recommendation letters.
In the first paragraph, briefly describe the experience gained by the individual you are recommending, and follow it by describing his qualities in the second paragraph.