CC: Carbon Copy
CC stands for carbon copy.
The purpose of CC is to include multiple recipients within an email.
When using CC, all of the recipients of that message will be visible to other recipients. Therefore, CC should only be used for internal communication, whereby the ability for recipients to view other email addresses would not be a breach of privacy.
If wanting to hide email addresses from other recipients, the sender should use BCC instead.
Caution should also be applied CC'ing people unnecessarily, since every email has a carbon footprint. Therefore, mass email sending or long email threads should be discouraged unless necessary.
Example
A manager wants to tag all employees within a message with information about an upcoming meeting. Since all recipients are known to each other, CC is appropriate to use.
Also, CC'ing avoids the need to email each recipient individually.