You can go to the Asus website and add the two boards to a compare list, then see the specs side by side.
The primary differences I saw at a casual glance is the M has a slightly better IGP, but the LX has a couple extra PCI slots.
http://www.asus.com/Motherboards/Intel_Socket_1155/P8Z77V_LX/
http://www.asus.com/Motherboards/Intel_Socket_1155/P8Z77M/