there are numerous differences and even differences amung the 8n's themselves. The 9n's were older, They had different paint colors, different tire sizes standard, and some other minor differences. I think the 9N was rated at a lower HP too. The 9n's and 2n's arent nearly as popular as the 8n's. Of the N's the 9 is the oldest from between 1939-1942. the 2n's ran from then until 1947 and the 8n's took over after that and ran until 1953.
Just amung the 8n's there were several differences. Front mount and side mount distributor, Rotating and non-rotating exhaust valves, .040 and .090 sleeves, 3-ring and 4-ring pistons, optional sherman trasmissions, just to name a few.
Why do you ask, are you looking at buying one??
I don't want to discourage anyone from posting on here but searches are your friends. Do a google search and you can read about them for weeks. There are also websites dedicated to the N-series fords. But your questions is like asking "whats the difference between a 1985 chevy silverado and a 2003 silverado". They're are many differences but still the same basic size/style of tractor. They had different options/features but are similar.
If you would like to know something more specific, we may better be able to assist you, so lay it on us. But do so in the vintage tractor forum as noted above.