The “Classic 3” - CIA

Confidentiality

Only authorized people can access the information.

Integrity

The information is correct, and cannot be corrupted by malicious actors. It may also include the concept of "origin" or "source" integrity - that is, can we verify that the information came from a specific source and has not been tampered with. The concept of "non-repudiation" also falls under the Integrity category.

Availability

The information is available to authorized users.

Physical Safety

In terms of the “classic 3” Physical Safety might perhaps fall under I. However it deserves special mention for some systems. For example, in an insulin pump, you don’t want an unauthorized actor to remotely change the dosage!

Conflicts

These attributes can sometimes be “in conflict”, or require “balancing” between the strength of the different attributes.

For example, in an electronic voting system, you want both anonymity(C) and non-repudiation(I).

Also, sometimes different groups of people will view the same attribute of “security” in different ways. For example, in the recent wikileaks/NSA saga, wikileaks might value anonymity(C) as an attribute of security as a way to protect its members and sources from reprisal. However, the NSA would take the same attribute of anonymity, and view it as a detriment to security, since it makes it more difficult for them to prevent future releases of classified information(C).