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?

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).