difference between lib and lib64 directories

[Kamaraju S Kusumanchi]

After compiling gcc on a x86_64 machine, I ended up with two similar looking
directories in the uppermost directory --- lib/ and lib64/.

What is the difference between these two directories? when should I use the
files in lib64/ and when should I use lib/? If there is a short document
where issues like this are explained in more detail, I appreciate if some
one can point that to me.

thanks
raju
-- 
Kamaraju S Kusumanchi
http://www.people.cornell.edu/pages/kk288/
http://malayamaarutham.blogspot.com/