basic kernel question

[Praveen Ray]

I always wondered what is the basic difference between major Linux distros? 
Does each vendor modify the stock kernel or only the packaging is different? 
Am I missing something (so far as kernel is considered) if I do not use RHAS 
and simply build my own kernel? Had RedHat somehow modified the kernel to 
make it 'better' or the only difference is that RHAS comes with all sort of 
peripheral software prepackaged?