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. Governance relates to decisions that define expectations, grant power, or verify performance.
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.
Examples or guidelines are:
The Federal Sentencing Guidelines are rules that set out a uniform sentencing policy for convicted defendants in the United States federal court system A medical guideline (also called a clinical guideline, clinical protocol or clinical practice guideline) is a document with the aim of guiding decisions Human interface guidelines (HIG are Software development documents which offer application developers a set of recommendations