The cover letter is what you attach to your CV when sending it for a possible internship. The cover letter is the first thing that the employer sees and the letter does help in his judgment of the candidate. So, as is obvious the cover letter should be impeccable and show you in the best light.
The cover letter should highlight your reason for wanting to pursue an internship with the company you have applied to. The purpose has to be clearly stated. Do not write fuzzy sentences in your cover letter. The sentences should not be very long. People tend to lose interest in long sentences.
The cover letter should give a little introduction to your CV. The introduction should be such that it highlights your suitability for the job. Do not mention details that are not related to the job you are applying for in the cover letter.
The key to a good cover letter is to be precise and accurate. The people who go through your letter have many such letters waiting to be read. A lengthy cover letter might not do a lot of good to your chances. Plus, the people at the company want to assess you on the basis of the cover letter too. You don’t want to hamper the assessment by writing a lengthy cover letter. Your accomplishments might get ignored as they focus on the irrelevant parts of the letter.
Please ensure that the grammar of your cover letter is correct. Incorrect grammar might create hindrance in reading your application. Also, many people are put off by bad grammar and might find it reason enough to push you on the back burner.
These little details should be kept in mind in order to write a good cover letter. Ultimately, it is the CV that would decide your suitability for the job, but a good cover letter would arouse the interest of the person reviewing it and might increase your chances of being chosen.