ECN stands for Electronic Communication Network whereas STP stands for Straight-Through-Processing. ECN and STP are two different methods by which no dealing desk (NDD) brokers operate, however, for a forex trader they will in most case effectively the same, if you are trading with a reputed & well regulated STP broker.
Essentially, ECN brokers connect you with other traders in the forex market so that you can affect your traders. This means that instead of trading with the broker themselves, you are trading with other traders on the market.
This helps you to avoid any conflict of interest with the forex broker. Also, the ECN method of brokering is cheaper and you will be charged a lower spread for the services of the broker.