C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 18:
[18.6] What does "Fred const& x" mean?

It means x aliases a Fred object, but x can't be used to change that Fred object.

Read it right-to-left: "x is a reference to a constant Fred."

For example, if class Fred has a const member function called inspect(), saying x.inspect() is OK. But if class Fred has a non-const member function called mutate(), saying x.mutate() is an error (the error is caught by the compiler; no run-time tests are done, which means const doesn't slow your program down).