ECN - (born ECN - Electronic Communications Network) - electronic trading systems.
ECN provides a trading venue where all parties (banks, market participants and traders) are traded against each other by offering prices within the system.
ECN - broker - a type of brokerage firm that provides clients with direct access to other participants in the currency market. ECN brokers do not restrict scalping, do not trade against the client, do not charge a spread (although, still, there are low spread is dictated by market prices) but charge a fixed fee for each order.
ECN trading has advantages compared to trade through brokers who are market makers, since ECN brokers deals traders are not enforced against the broker, and have their own counter-agent, or another client ECN broker, or in the form of liquidity provided by the partner ECN broker.
STP (born STP - Straight Through Processing) - from beginning to end automated processing of transactions and (or) payment orders, including the automatic creation, adoption of the regulations, clearing and settlement.