Some are ATi, some Intel, Some NVIDIA, Some VIA
ATI chipsets will let you use CrossFire, very stable and good boards.
Intel chipsets will let you use CrossFire too and generally are the most stable boards
NVIDIA chipsets will let you use SLI and the last 680i boards have reported to be very very good boards
VIA chipsets, dont really know, I dont use them. I cant say anything about them.
Now if you mean the differences between 965, 975x, 680i, RD600 etc. There are not too many differences, you will almost find the same features on most of them. Main difference will be support for either Crossfire or SLI. The rest is almost the same.