Cover Letter (CL) is a document that accompanies your CV when applying for a job. As the CV is a document written according to the established pattern and thus is similar from candidate to candidate, a cover letter is your chance to give some individual stamp to your biography and to stand out your specifics. CL is preferably as short as possible (200-250 words), and its aim is to guide the employer to the CV, to show the activity and characteristics that are of particular importance for the application.
Your cover letter should answer the questions that the employer might ask himself reading your CV. In short, the goal of the cover letter is to answer the question of employer: Why should I hire this person?