The briefing is an essential part of any mission. It usually displays a synopisis of what to do. Hence, briefing.
Briefing.html is a simple HTML file located in the mission folder. You have to create this file manually and then the file will be loaded automatically as the mission begins.
BIStudio has provided an overview of the different aspects of the briefing.html file located here.
The briefing.html is broken into 4 main sections.
To "preview" your briefing whilst in the editor, hold CTRL+SHIFT while clicking "preview".
The original BIS style briefings employ a small amount of information on the mission to be played, giving the player only the most basic of intelligence needed to complete his task. The BIS notes section usually takes the form of a diary entry and whilst these methods are certainly an accepted in the community, there is another way to write your briefings.
The advanced briefing takes writing and presenting a briefing to a whole new level. Using links and pictures you can create a much more detailed and aesthetically pleasing effect which will allow you to share with the player, as much information as you wish. Multiple pages will allow you to present different portions of your briefing in an orderly fashion. For example, you may wish to break your briefing down into phases such as:
Sitrep (Situation Report) Orders Insertion Target Exfiltration Friendly Forces Enemy Forces
With various links this is easily achievable.