i386 vs SRPMS iso image difference? - newbie question

[David Hunt]

I have downloaded and installed Fedora using the i386 iso images. That 
worked very nicely.
I also downloaded the SRPMS images, but didn't appear to need them. What 
are the SRPMS images for?
Can anyone really quickly tell me the difference between the i386 and 
SRPMS iso images?

Also, how do I know what kind of processor I've got?
I am running an Intel Pentium 4 2.66GHz processor. Is that i386, i486, 
i586, or i686?
Would it make any difference to try recompiling the kernel for the 
correct processor type?
If so, dare I ask how that is done?

Thanks,

David Hunt