Basically it indicates the machine architecture that particular package is compiled for, its not backward compatible so if you have a 386 cpu(doubt there's many of them around though) only i386 package will work, but if you have a pentiumpro(i686) or above all of them will work.
Genererally speaking the higher the machine architecture used to compile the package the higher performance you'll get.
Machine architectures for modern CPUs like athlon-xp or pentium4 can also be used(since gcc3 or something, not sure) so you'll be able to get optimized performance if you compile your packages from sources with proper cflags. However major distros such as redhat, mandrake etc don't use them in their packages for the sake of compatibility.
Last edited by Demonbane; 10-29-2003 at 06:26 AM.