Writing a cover letter is about creating a connection with the recruiter so that the recruiter sees enough potential in your candidacy to call you in for an interview. The cover letter is not to be written in isolation, but in connection with your resume. The two complete one another.
When writing your cover letter, look at it as an introduction of who you are, what you can bring to the position, and why you are interested in the position. If what you write makes sense, is compelling enough, and is corroborated by your resume, you may be given the opportunity to meet with the recruiter in person. It’s hard to know what is “compelling enough” to a recruiter, and each person probably has his or her own view on this, but it remains that the work environment is about the actual work, but also about the people dynamics. Keep this in mind. You must show that you can do the work, but also that you are someone that works well with others. How to convey this takes many drafts and is part of the “art” of writing a good cover letter. One that is authentic, yet conveys the relevant information in a compelling way.
Last but not least, make sure to tailor your cover letter for each position you apply to.