A query letter is basically an inquiry note from the writer to a publisher. In such type of correspondence, writers present their idea, give synopsis or offer the whole script and ask the publishers whether they are interested to publish it or not. However, you must know that the agents get countless proposals everyday so you should send a well written query letter to lock their attention.
Remember, it is your best shot and you do not want it to be your last. To make this effort more fruitful, you will have to keep couple of important things in mind.
First of all, you will mention your name, address and other details in separate lines. After that, mention the date of submission followed by the name and address of the recipient. Give a suitable salutation and then start your letter with a courteous sentence. Before coming to the point, make your addressee realise that you admire his commitment to the profession and that forced you to send the proposal to him. Next paragraphs of your letter will comprise a synopsis of the story.
However, you must know that the purpose of this description is just to make it more understandable. You are not supposed to send complete story with all turns and twists. In the end, you should remind them to check the attachments. Make them realise that you are ready to make few changes if needed. Conclude your letter with a proper greeting followed by your name.