Spread
The spread is the gap between the ask (buy price) and the bid (sell price). It is a cost you pay every time you open a position.
If you buy at the ask and sell straight back at the bid, you lose exactly the spread. It is the main income of commission-free brokers, and part of the cost at those that charge commission.
Spreads can be fixed or variable. Variable spreads are tight when the market is liquid, for example on EUR/USD during the London–New York overlap, and widen during news releases, overnight or at the Sunday evening open.
To compare two brokers, add spread and commission together, in pips or in money per lot. A 0.1-pip spread with $7 round-turn commission works out at roughly 0.8 pip in total.