We often hear about security, but very often we do not receive a clear definition of what this is, since it's taken for granted. Even if we know what security in general is, sometimes we can miss some pieces of what security means in that specific field. I, personally, like to use this definition of information security—preservation of confidentiality, integrity, and availability of information.
The ISO/IEC 27000:2009 affirms that "In addition, other properties, such as authenticity, accountability, nonrepudiation, and reliability can also be involved."
This highlights the fact that security is a very wide sector, including two very different realms:
Data protection from unauthorized access (confidentiality)
Data integrity and availability
Before we dive in the security realm, we need to look at some important concepts of security.