AM3+ Socket Type

Here are the differences I know of... there may be more
1) 990FX supports 2each PCIe lanes at x16 or 4each PCIe lanes at x8, can run 4 PCIe cards 970A supports only one PCIe x16 or 1each x16 + 1each x4 and cannot support more than 2 cards
2) Higher motherboard wattage, 990FX=~19W vs 970A=~14W

That's all I am aware of