When you send your curriculum you can/have to attach also the cover letter on the same email. Use this letter to underline your professional goals and try to explain why you want to work in that company and tell them why you are good for the position they are looking for. Remember that this letter has to be personalized based on the company receving it and also based on the role they are looking for.
Be sintetic. Sintetic but tell them what they need to know. Send a curriculum without the cover letter is a mistake: try to avoid to give the company the sensation that you already sent the same cv to many other companies. They should feel like you have been considering to them and the position they are offering as a special occasion. You should look special! Give them the sensation they really really need you ;) Letting them think that we thought long time about this position and we are right because of our professional career or what we have been studying till now. Now we are ready to start from this point. From the role they are offering in their company.
So, a short text to tell where we are now and where we want to go. And why we are the right person. Then let the curriculum vitae speak.