there is a good article to explain ithttp://en.wikipedia.org/wiki/LGA_1155
in simple words
h61 and b75 are chipsets for lga1155 cpu socket of intel
b75 is much better than h61 due to native support for 2nd/3rd gen core series cpu, native 1600mhz ram support with 3rd gen cpu, native sata 3, usb3, pci-e v3 support, it also support base clock overclocking
purchasing decesion of either h61 or b75 depends on your purpose and your cpu , and budget too so provide some details here .