An order book is the real-time list of all outstanding buy and sell orders for an instrument, organized by price level, that a liquidity venue uses to match trades. It's the raw data behind market depth — the deeper and more balanced the book, the more liquid and stable the price tends to be.
Most retail forex brokers don't expose a full order book, but some ECN and DMA platforms show a simplified version so traders can see where orders are stacking up.
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Guide
Order Types and Slippage Explained
Market, limit and stop orders trade price certainty against execution certainty — plus what slippage is, where it comes from, and what a stop-loss caps.