Operations Security, or OPSEC, is the process by which we protect unclassified information that can be used against us. OPSEC challenges us to look at ourselves through the eyes of an adversary (individuals, groups, countries, organizations). Essentially, anyone who can harm people, resources, or mission is an adversary.
OPSEC is used to protect information, and thereby deny the adversary the ability to act. Nearly 90 percent of the information collected comes from 'Open Sources.' Any information obtained freely, without breaking the law, is Open Source. They include social network sites, tweets, text messages, blogs, videos, photos, GPS mapping, newsletters, magazine or newspaper articles, your college thesis, or anything else that is publicly available.