It’s pretty self explanatory. A Buy order refers to an individual attempting to Buy a particular coin, in exchange for another coin. A Sell order is the opposite. But realistically the entire transaction is more appropriately caller an Exchange, since each party is both a buyer and a seller.
GateHub is a little more complicated. You first have to connect to a Gateway and then choose what you want to trade with. Then go to the Trade tab and select which two coins you want to trade between. You you can choose to buy or sell by clicking on either buy or sell. Binance is similar to Poloniex in that you choose your market tab in the top right of the screen. You can choose between BTC, ETH, BNB, and USDT. For instance, if you have the BTC tab selected then just below the tab is the pairings which you can scroll through. Once you find the pairing you want (eg. DASH/BTC) select it. You'll then see the chart change and the pairing indicated on the top left side of the screen as DASH/BTC. You are now in the DASH/BTC pairing and can now use the Buy or Sell functions below by placing a Limit, Market, or Stop-Limt order. Just to the left of that is the active order book where you'll see who is buying and selling. You can either match a current offer, generally by clicking on it and it should auto-populate in the buy or sell price-then just make sure to adjust the amount you want to or sell and then place the order; or you can put in your own price offer and if the price threshold is met then the order will execute, if not it will stay either until those conditions are met or until you cancel the order.