Bitcoin Exchanges (USD)
Bitcoin exchanges allow you to buy or sell bitcoins, the ask price is the lowest price a seller is willing to accept for their bitcoin. The bid price is the highest price a buyer is willing to pay for your bitcoin, you can click the bid price column heading to re-order the bitcoin exchanges by bid price. Spread is the difference between the ask and bid prices.
Last price shows the price of the last completed bitcoin trade, this could be either a buy or a sell. You should also take into consideration the exchange volume (number of bitcoins traded within the last 24-hour period) as an indication of how recently the last price (i.e. last transaction) was executed. You can re-order the bitcoin exchanges by volume by clicking on the volume heading. It's best to use an exchange with high liquidly (i.e. volume) to ensure your order will be executed quickly.
If you hover over any of the prices in the table you will be shown the price as a percentage relative to the current Bitcoin Price Index (BPI) which is provided by and displayed in the menu bar above. The Bitcoin Price Index is an exchange rate calculated as an average of bitcoin prices taken from a number of leading global bitcoin exchanges.