ECN stands for electronic communication network. STP means Straight-Trough Processing (meaning no dealer intervention for your trades). ECN Forex brokers provide you with direct access to other participants in the forex/currencies market - both retail and institutional traders, news about which you can get at Market Watch Currencies. This leads to some advantages, e.g. ECN brokers allow scalping and offer very low spreads, but there are also disadvantages - these brokers charge commissions for forex trading. Generally speaking, trading through ECN forex brokers is more transparent, as you see real exchange rates and know how much your broker makes(a commission of, say, $3 per standard lot).