C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 14:
14.1 What is a friend?
14.2 Do friends violate encapsulation?
14.3 Pros/const of usign friend functions?
14.4 Meaning of "friendship isn't inherited, transitive, or reciprocal"?
14.5 Declaring a member function or a friend function?
[14.1] What is a friend?

Something to allow your class to grant access to another class or function.

Friends can be either functions or other classes. A class grants access privileges to its friends. Normally a developer has political and technical control over both the friend and member functions of a class (else you may need to get permission from the owner of the other pieces when you want to update your own class).