An ECN broker is a pure intermediary. Your order is matched against other participants in an electronic liquidity network — banks, prime brokers, other retail traders, market makers — without the broker taking the other side themselves. Spreads are therefore as tight as the underlying market allows, often 0.0–0.2 pips on EUR/USD during London, but you pay a commission (typically $3–$7 per standard lot per side).
ECN is usually the right choice for high-volume or scalping strategies where the spread matters more than the commission, and for anyone who wants zero conflict of interest with their broker. Standard accounts — with wider spreads and no commission — are often cheaper for casual traders who place few trades per day.