Environmental threats are facets of your environment that pose a security risk. If out on the town, environmental threats might include a poorly lit alley or the kind of crowd through which you're moving. An environmental threat for a secure facility might be a tree with a branch that grows past a perimeter fence.
Awareness is the best safeguard against environmental threats. Recognizing them is the first step in forming a plan to meet and mitigate the problem.
Physical threats are real and often immediate threats to your well-being, the structure of a facility or to maintaining possession of an asset. A mugger is an example of a physical threat on your person, as is an oncoming car.
Physical threats are best dealt with via direct action. The safest personal response is to leave the area and call the authorities. If you cannot safely do this or live in an area where authorities aren't responsive, you may require the services of a private security firm.
Information threats are "leaks" of knowledge that could be used to create a security breach. At the national security level, these threats are dealt with via active anti-hacking teams and by keeping sensitive information in the hands of trustworthy personnel. At the personal level, you handle information threats by keeping your passwords and pin numbers secret. If you fear there has been an information security breach in your area, you should notify everybody concerned and immediately change as much information as you can to render the leaked information useless.
A passive threat is a threat that hasn't yet happened - an opportunity for somebody to take advantage. Some environmental threats are also passive threats, as are breaches in security protocols for information or physical security. Security consultants make their living by identifying passive security threats and recommending ways to alleviate them, such as better lighting, security cameras and altered business protocols.
An aggressive threat is somebody actively attempting to breach your security, such a a hacker, burglar or attacker. On-site security is the best countermeasure for facilities at a high risk for aggressive threats. For everybody else, the safest course of action is identical to your response to a physical threat: Leave the area as quickly as possible and immediately notify the appropriate authorities.