Information security is the protection of information from a wide range of threats

in order to

• ensure business continuity
• minimize business risk
• and maximize return on investments and business opportunities

Information can exist in many forms:
- printed, electronic, shown in films, spoken in conversation etc.

Three aspects of security generally have to be considered;

• confidentiality
• integrity
• and availability

This is often summarized as giving the right information (integrity) to the right party (confidentiality) at the right time (availability).

Inferred aspects are

• privacy
• anonymity
• and verifiability

Confidentiality (UK Government classification):
RESTRICTED, CONFIDENTIAL, SECRET and TOP SECRET

It is now more generally recognised that a lot of critical information may be widely available, but still must be correct, for example the prices displayed on an on-line shopping site or corporate accounts, or must be available when required such as emergency response plans.

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References
http://en.wikipedia.org/wiki/Information_security