Nicehash is a bitcoin mining hash reseller. You could download their client and mine whatever cryptocoin was most profitable (ETH/BTC/LTC...) and get paid in bitcoin. Actually quite easy to use.

But they just got hacked for $52million...

No, it's a mining pool. If you have a miner, you can direct it to Nicehash, and it gives you a proportion of all the pool's mining rewards. This way, it reduces the variance of rewards for individual miners, who may otherwise go months before mining an actual block.

It's not quite a mining pool. You don't solve shares and get it back tied to that share.

You sell computing power for $x/hr, and you get paid $x-%/hr, as two completely unlinked things.

Their rates were about 4 BTC to rent 1 TeraHash/s of mining capacity for 24 hours. Some of their pools for rent had 1800 miners, some were solo operations. I was looking into renting mining time through them a few days ago.

At this price rate, the ~4000 BTC could be 1 or 2 days of rental fee for a couple hundred transactions. No idea about their volume of transactions or cash flow.

A multi-pool: you point your miner at them and they switch automatically to the coin with the highest expected profit to mine. They pay miners in BTC. On the other side, they let people bid on mining power to direct to coins of their choice.

As others have pointed out, it's not a pool. They're a service for connecting people with miners to people who want to mine something. They're a hash reseller. A user bids for mining power and where they want it assigned. Miners post rates they're willing to mine at.