There are two types of threat categories: natural and human threats.
A natural threat is just what it sounds like; its source is either from nature or a systems environment. Natural threats can include lightning, fires, hurricanes, tornadoes, or floods. Threats to a systems environment can include poor building wiring or insufficient cooling for the systems.
Human threats are caused through unintentional or intentional actions. An unintentional threat is a human accident, bad habit, carelessness, or misinformation. An intentional threat, whether caused by an insider or outsider, can be a spy, hacker, corporate raider, or a disgruntled employee.
The insider intentional threat is one of the most challenging security problems today. Since insiders have working knowledge of and access to their organizations computer resources, the potential for damage is great. One example could be someone you work with who is extremely disgruntled. Who is to say he or she wont misuse your system to delete or change sensitive information?
Have you ever accidentally spilled coffee or soda on your keyboard or computer? This unintentional threat occurs more than any other type.