Stock indices, like the NASDAQ and NYSE in the US, the DAX in Germany, or FTSE in the UK, are popular among independent traders who typically trade them in the form of contracts-for-difference (CFDs) from ECN forex brokers and others.
What separates the ECN brokerage model from other brokerage models is that ECN brokers don’t match their clients’ orders internally, but instead sends them directly to an electronic communication network (ECN) where they are matched with orders from other traders. In this sense, we can say that ECN traders are trading against other traders, while traders at dealing desk brokers are trading against their own broker.