i386 and i686

Well, I was browsing http://www.linuxiso.org when I became aware that I had no idea of the difference between i386 and i686. I know intel processors are i386/586 and that AMD is i686, but could someone tell me the difference between the two as far as programs and such? I want to know before I try and instal the i386 ISO of Gentoo on my machine, which is an i686

If you have a version that is for i386, it is not optimized for later processors,
but is fully compatible back to a 386. If you know that you will never need
to install on a 386, use the version that is optimized for your processor.
:cool: