You don't send a script out unless someone has requested it - either through a pitch session, in response to your query letter or because your cousin is best friends with the head of development at Paramount.
At that point, you write a cover letter based on the relationship you have to the person you are submitting it to, ie. formal or less formal. In most cases you will need to remind them that they requested your script and say something about it to jog their memory as to what it's about, etc. If it's someone you know you can get away with an even less formal handwritten buck slip.
In any case be brief and polite.