Whats the difference between i386,i686 etc processors?

i386 is for extremely old CPU generation dating before Pentium.

i686 is post Pentium generation.

No one today have i386, they are at least 15 years old.

That being said, i386 marks a 'compatibility' build and should work on ANY 32bit x86 CPU. i686 may or may not use MMX, SSE and more extensions.