The interbank market is the decentralized network of large banks that trade currencies directly with one another in vast volumes, setting the reference rates the rest of the forex market prices off. It has no central exchange — deals happen bilaterally or through electronic brokering systems like EBS and Reuters Matching.
Retail traders never access the interbank market directly; brokers source liquidity from it (or from prime brokers who do) and pass on a marked-up price.
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Guide
How Execution Models Work: ECN, STP and Market Maker
Market maker, STP and ECN describe where the other side of a trade comes from — and what each model changes about spreads, requotes and order handling.