i386 means the package is compiled for generic i386 based machines. Which includes all 486's, Pentiums, Pentium Pro's, Athlons, Pentium 4's, and anything else with the x86 architecture.
i486 packages mean they have some optimizations that mean that will only work on i486 and higher (so you are out of luck if you still have that old 386 kicking around).
i586 is Pentium 1 class machines (and AMD K5's) and higher
i686 is Pentium II/III class machines and higher