The CIA Triad
Confidentiality is, as the word implies, having something be confidential or secure. In essence, privacy is security and confidentiality means that third party individuals cannot read information if they do not have access to it. Data to think about keeping confidential is data stored on a computer (temporary data, data saved, etc.), data stored for backup, data in transit, and data intended for another person. Confidentiality will be the main focus point of this article as it is most often referred to as the most important aspect of security.
The I in CIA stands for Integrity and is specifically referring to data integrity. Integrity is the act of ensuring that data was not modified or deleted by parties that are not authorized to do so. It also ensures that if the data was changed, that the authorized person can know about it. Simply, if you send a message to someone, you want to make sure that the person does not receive a message that was altered during transit. Integrity also confirms that you are in fact speaking to who you think you are speaking to (for example: we download an add-on from the website, you want to make sure that you are downloading from that website and not an unscrupulous third-party).
Finally, the A stands for Availability and ensures that when you need the data it is available to you. Not only does data have to be available to you, but it has to be reasonably accessible. There's no point in security controls if you cannot access the data! This component is a concern, but for the average end user, there is not much that can be done to ensure availability when dealing with webpages, or IRC servers or anything else managed by a third party host. For this reason we will not be discussing Availability except for backing up your data in this guide.