The script for a motion picture. The screenplay tells the actors what to do, what to say, and (if it's a good screenplay) why. Many screenplays also include a small amount of camera instruction, but a good screenplay will try to leave most of that sort of thing up to the director and cinematographer.

There is no concrete format for a screenplay, and thus most of them look slightly different. However, it is generally accepted that screenplays are written in the present tense and follow a basic set of formatting guidelines.

First, you must set the scene. This is usually done with one line, like this:

1. EXT. PARK - DAY - RAINING

The first thing on the line is a scene number. Not all screenplays use these, but I've found they can be very useful in keeping organized during shooting and editing. "EXT." tells us that the scene is outside. If the scene were inside, we would use "INT." "PARK" is the location of the scene. The location can be general, like in this example, or it can be specific, like "CENTRAL PARK". After the location is the time of the scene, which in this case is day. If the scene needs to be shot in stormy weather or during a specific season, we specify that as well.

Next, we need to describe the scene. This is done like so:

We're in a park. It's a nice one, with TREES and BENCHES and a POND with a
FOUNTAIN in the middle. There are DUCKS doing whatever it is that ducks do
in the rain. Nobody is in the park, because nobody goes to parks when it
rains.

MOWRY walks into frame from the left. He is walking away from us, following
a PATH that meanders close to the pond, then past a bench. We follow him as
he follows the path. When he reaches the bench, he pauses, facing the pond.
He looks at the fountain for a moment. He glances upward slightly, thinking
about the rain. He sits. He thinks. His expression is one of calmness.

As he sits and thinks, we hear a noise from further down the path. It's an
OLD MAN, frail, holding a LEASH. Attached to the leash is a SMALL DOG. The
man is walking casually behind the dog, humming to himself. He approaches
the bench. Mowry gives no sign of having noticed him.

The old man sits slowly, as if he must be careful not to adjust his posture
too quickly or his joints will be unable to handle the stress. He grunts as
he sits; a grunt that signifies his transformation from a working state to a
relaxing state. The grunt is followed by a sigh, which is his way of showing
us that there is nothing at all he would rather be doing right now than
sitting here on this bench, in this park, in the rain.

The dog sniffs happily at Mowry's shoes.

When the word "we" is used in a screenplay, it is used in the sense that "we" are the audience or, in some cases, the camera. The words in all-caps are things that will require the attention of the prop department or roles that will need to be cast. Not all scripts use this convention, and some that do don't use it consistently, but I find it helpful in preparing to shoot a scene.

Next, we have dialog:

					OLD MAN
		Beautiful weather.

Mowry looks at him, but his expression does not change.

					OLD MAN
		I love the rain. The smell. (gestures at the pond) The way it seems
		to merge with the pond when it hits. Gives the world atmosphere.
		And...purpose.

Mowry is cupping his hands, letting pools of rainwater collect before
dumping them on the dog at his feet. The dog doesn't seem to care.

					MOWRY
		I don't mind it.

They both watch the raindrops impacting the surface of the pond.

					OLD MAN
		It's nice to see someone else enjoying the rain, for once. Rains
		often enough around here you'd think people wouldn't be afraid of
		it, but they are. Feel a few drops on their heads and they run for
		cover like it'd kill them to get a little wet. As soon as the sun
		comes out, they're pissing their swimsuits in the nearest pool.

					MOWRY
		The world doesn't make a lot of sense.

					OLD MAN
		Ain't that the damn truth.

This shouldn't require a lot of explanation. The formatting here is up to you, but in general the name of the person speaking is written in all-caps and followed by a new line containing their dialog. If you have a long section of dialog spoken by one character, you can split it up into paragraphs (it helps in this situation if you offset the dialog from the directions by a few tab widths).

As a general rule, you can estimate the running length of a film by the number of pages in the screenplay. One page equals roughly one minute of screen-time. Thus, most screenwriters try to keep their screenplays shorter than 120 pages or so.

There, that's it. Those are the basics of writing a screenplay. Have at it, and have fun.

Don't believe a word I've written here. This is just a very simplistic overview of how I write my screenplays for my silly little goof-off films, and you'll probably get laughed at in Hollywood. Or even Bollywood. Instead, go pick up a copy of David Trottier's Screen Writer's Bible. It comes highly recommended by LdrWilhelm. David Mamet's book On Directing Film also contains some good general screenwriting advice (despite focusing mainly on direction).