Recommended practices published by an OEM, government, or other organization (e.g. Apple Human Interface Guidelines)
A guideline is any document that aims to streamline particular processes according to a set routine. By definition, following a guideline is never mandatory (protocol would be a better term for a mandatory procedure). Guidelines are an essential part of the larger process of governance.
Guidelines may be issued by and used by any organization (governmental or private) to make the actions of its employees or divisions more predictable, and presumably of higher quality.
Reference http://en.wikipedia.org/wiki/Guideline, retreived 2011-04-25
A list of guidelines can be found here:
Creating a 'Guidelines' Questions
Do check the existing list of guidelines first.