Well if you wanted to buy a workstation computer it could really have any motherboard you wanted as long as it has the functions you need, ie. a pci-e for a graphics design computer. A server on the other hand is not designed to do work in a sense, it will usually host files for websites or whatnot so it will have a different south bridge with more controllers for many hard drives and probably more dimms, it will also not have overclocking features wheras a high end workstation board can.
so really the essential difference is that a server board is not made for a computer that will be doing work, it will be made for a computer that might need high amounts of memory and hard drives connected to it, i think you will find some might not even have a pci-e slot or onboard audio etc, it just depends on the board. A workstation board might also support higher amount of ram than normal but not as much as servers, a workstation will probably have more expansion slots too. at least this is my understanding, someone else probably knows a lot more accurate info.