A selling paragraph. A paragraph which convinces the recipient that if they don't read your script, they are going to miss out on the script that will make their career. Only you don't say that in your letter. Always research the recipient of your query letter. The query letter is a written pitch.
Query letters are always a sales tool.
Query letters are usually written by the writer to a producer or executive at a production company or to a literary agent. Queries are single-spaced, professional business letters. One page in length only. Avoid stationery that is jokey, clever, or precious. Always send the letter to a person not to a company.