processing speed

Ok, I know I am in the newbie section for a reason.....

I am confused on the processing speed.

Can someone explain to me how is the hashing speed calculated?

For instance a KNC Miner Neptune has a 3000 GH/s hashing speed, how is this calculated? My thought process is around could I put together a series of processing cards to achieve the same thing?

For instance if a dell computer has a 3.9 GHz processor does that mean it is 300 times slower than the KNC Neptune? How is KNC Neptune putting some much speed in such a compact area? It seems they are also using the same chip that is in an apple iphone? What makes the Neptune so unique?

Could someone put together a large number of small processors to achieve the same processing rate or better somewhat like a supercomputer?

I would appreciate some help here.