The VXX, sometimes called a VIX stock, is the ticker symbol for the iPath S&P 500 VIX Short-Term Futures ETN. This is not a stock but an Exchange Traded Note (ETN) that you can trade like a stock.
It is issued by Barclays Capital and is one of the largest and most successful VIX products. The previous ticker was VXXB.
The VXX tracks the S&P 500 VIX Short-Term Futures Index Total Return, which provides exposure to a daily rolling long position in the first and second month VIX Futures contracts.