Spread
The spread is the gap between bid and ask price. It is the main cost of trading a forex pair.
Every forex quote has two prices: the bid (what the broker pays you to sell) and the ask (what you pay to buy). The spread is the difference between them, expressed in pips. On EUR/USD a tight spread is 0.0–0.3 pips on Raw / ECN accounts and 1.0–1.8 pips on Standard accounts. Spreads widen during low-liquidity sessions (Asian open) and around high-impact news (NFP, CPI, FOMC). A broker that quotes a 0.0 pip spread usually charges commission separately. A broker that quotes spreads only usually marks them up to bundle the commission.
Worked example
XM Standard EUR/USD quoted at 1.0850 / 1.0852 has a 2-pip spread. On a 1-lot trade that is a $20 round-turn cost before the price has moved.