It is no longer mandatory to send a traditional cover letter along with your CV. Do so only if it is required or you consider it to be appropriate. Most of the time, an e-mail with your CV as an attachment will suffice. The body of your e-mail should be a short, informal cover letter.
A cover letter should be specific and concise, no longer than one page. You must include your career objectives in relation to the position, what interests you about the proposed tasks and a short summary of your experience and skills.