It is a crypto exchange that has decided to lower its trading commissions. They calculate your 30-day transaction volume in terms of dollar equivalent of your asset, not in btc as before. Competition is good.
There are trading pairs available on Poloniex. However, they don't list every coin.
The support team responds very quickly. Team is oline 7/24. Poloniex mobile app is not bad. However, there is a lot to be improved. I have a 3-year membership, but I cannot say I prefer it very much. The exchange also has futures trade options.