An example is if you have a website that you maintain for your family and friends that you are away from. Is it password protected or can anyone surf on and gather vital information about you, your family, your spouse and even your military installation? One article I've read has talked about pictures of installations on family websites were posing a significant security risk, along with information on units and their movements.
Sometimes this just seems all too much, but it is very important that we all think twice before we reveal information in a public forum. I know sometimes it just seems a little far-fetched that someone would monitor a support board, but consider this - people who want the information (and there are people who do) are going to use any means possible to get it, why make it easy for them.
Simply put, OPSEC is all about denying the bad guys information to protect personnel and maximize mission effectiveness.
Anti-terrorism relies heavily on OPSEC. When personnel identify and protect critical information (military operations, capabilities, limitations, intentions, personnel, programs), OPSEC becomes a proactive means by which adversaries are denied this important advantage.
All servicemembers possess critical information the adversary wants. References to this information are maintained on a critical information list, commonly known as a CIL. The lists should be kept handy, for convenient reference. Unit OPSEC managers are responsible for tailoring the CIL to their particular work areas and distributing it throughout their units.
If a person needs to discuss critical information, they should use a secure telephone. Government telephones are subject to monitoring at all times. Cell phones, while extremely convenient, are one of the easiest means of communication to monitor. No critical information should be discussed on a cell phone. People should also be careful of what they throw in the trash because "one man's trash is another man's treasure".
While there is no critical information list for family members, they should stop to consider if what they are talking about would be of interest to someone collecting information. Everyone has a role in OPSEC, and that includes family members.
Information such as the cancellation of leave or work schedules can provide the adversary with another piece of the overall puzzle he's trying to piece together.
From Military.com