My new motherboard is 10/100/1000 and my old PC is a 10/100, whats the difference, I know a Mathematician will say a 1000, but in kbs it might
be different.

Probably referring to the LAN port. Your new motherboard is capable of 10mbps, 100mbps and 1gbps whereas your old PC is not capable of 1gbps.

And depending on the switch or router you hook your old pc up to, your other pc while capable of 1gbit, will in fact be stuck at 100mbit. Just like the ol wireless issue of G vs B.