One of the easiest ways to start is to look at nicehash.com and hashpower.co
Sit down, take cold beer and look at what is going on. Why is someone buying hash and what he/ she does with it. You will soon figure it out.
For the Ethereum mining with Nicehash, Nicehash itself is the one buying the hash. We do not know what they do with it.
I think they are the middle man. They buy hashes @ low and sell hashes @ high. Profit . No ?
For other algorithms such as X11, NeoScrypt or Quark, there are markets. But for Ethereum, it is just pool mining there.