General Orders and Special Orders govern a US soldier's actions while on guard duty. Special orders vary depending on the nature of the guard duty. There are three general orders, and they remain constant. Every soldier is expected to know his three general orders and be able to recite them on command. They have been ingrained into every soldier from the beginning of BCT.

General Order #1:
I will guard everything within the limits of my post and quit my post only when properly relieved.
Translation: You are responsible for everything that is included in your duty area for the whole time you are on duty. You must investigate suspicious activity, apprehend suspicious persons using (only the) necessary force.

General Order #2:
I will obey my special orders and perform all my duties in a military manner.
Translation: You must be familiar with your special orders and any other orders received from commanding officers. Also it covers performing duties 'in a military manner'- in other words effectively and courteously.

General Order #3:
I will report violations of my orders, emergencies, and anything not covered in my instructions to the commander of the relief.
Translation: Appropriate action must be taken in the case of a fire or other emergency, or a situation specified in your special orders.

Source: TRADOC Pamphlet 600-4: IET Soldier's Handbook; aka SMARTBook

Disclaimer: By saying 'soldier', I, like most military members, mean a member of the United States Army. Likewise, airmen (and airwomen) refer to members of the US Air Force, sailors to the US Navy, and marines you can figure out on your own.