A document is the space in which you create artwork. In Illustrator, you can create documents destined for many different types of output. (For information on creating Illustrator documents for video production, see the Enhancing Video Production PDF at www.adobe.com/go/learn_ai_video.
You start a new document by choosing a new document profile based on your intended output. Each profile includes preset values for size, color mode, units, orientation, transparency, and resolution. All use one artboard, by default. For example, the Video And Film Document profile uses pixels instead of points, and you can choose a device-specific crop area, such as NTSC DV Widescreen, to create a document in the exact dimensions required, with video-safe guides in place to help you lay out your design for optimal viewing.