A full quotation is made up of 2 prices called the Bid and the Ask. The difference between these two prices is referred to as the 'spread'. The spread is essentially the profit a broker or bank makes for you to enter the trade (your transactional cost). The wider the spread the more expensive it is for you to trade, whereas the thinner the spread the cheaper it is to enter the trade.
Large and frequently traded currencies usually enjoy a small bid-ask spread while small and infrequently used currencies have a large bid-ask spread.
The spread becomes more important to traders who trade frequently, such as an intraday traders or scalpers. However the spread is less important the higher the timeframe you trade.
Here you can see the bid-ask spread when you open a deal ticket within MT4. You wish to go long (buy) EURUSD and open an order window
In both circumstances your entry price on your P/L (profit loss) will be negative as the spread has been attached to your order. The market will have to move in your favour by the distance of the spread in order for your P/L to become zero.