Broadly speaking, computer security means that your computer behaves as you want it to, and that your information is used as you intend. There are always risks, and always costs associated with reducing those risks. Although technology is intimately involved, in the end it is a business decision as to how much money, time, and convenience you will spend to reduce the risk. The risk is never zero, but if you ignore it, neither will it be small.
Security is a process, not an end-point. Risks change, data changes and the costs of securing processes change. There is no point in either over-protecting or under-protecting, so the first step is to assess the risks.
You can learn more about computer security by visiting awareness.security.uic.edu.