ECNs introduced anonymous, fully electronic, all-to-all matching and the maker-taker fee model that still shapes routing today. Several were absorbed into the big exchange groups; some survive as ECNs in equities and in currency markets.
In retail platforms, "ECN account" usually means raw spreads plus commission rather than a marked-up dealer spread, which is a different use of the same letters.
Example: an equities ECN charges 0.30 cents per share to take and pays 0.20 to add, netting 0.10 to the venue. On 100,000 shares taken that is $300 in fees, which is why passive execution and post-only-orders are worth real money to active traders.
Related: lit-market, alternative-trading-system