STP
STP (Straight Through Processing) describes a broker that automatically routes your orders to liquidity providers, with no manual dealing desk.
An STP broker passes your orders straight to one or more banks or liquidity providers. It usually adds a small markup to the spread it receives instead of charging a separate commission.
The point is to avoid the broker systematically being your counterparty. Execution quality then depends on the number and quality of its providers: the more there are, the more competitive the prices.
In practice, many brokers run a hybrid model: some flow is handled internally, the rest is passed to the market. What matters to you is measured real costs, observed slippage and regulation.
See also