Bittrex is an
altcoin exchange. You can exchange between many different cryptocurrencies with other users of the service.
The prices are determined in a peer-to-peer way: you choose what price you are willing to buy or sell for and someone else needs to match your price. The bid price is the highest price that someone is willing to pay for your cryptocurrency when you sell, while the ask price is the lowest price that someone is willing to sell you a cryptocurrency for when you buy. The last price is the price that was most recently used in a trade.
Is it a wallet?
No. Bittrex controls your coins when you deposit them, and you essentially have a "promise" from them that they will allow you to withdraw those coins from them.
dollar pair.
There is no dollar pair at Bittrex. There is the altcoin USD Tether, which has a company promising to back the tokens 1:1 with dollars, but there is no guarantee that you will always be able to redeem USDT for dollars.
You also cannot withdraw or deposit fiat.