CFD (contract for difference)
A CFD is a contract that pays you, or charges you, the change in an asset's price between opening and closing, without you owning the asset.
With a CFD, you don't hold the share, the bar of gold or the index: you enter a contract with your broker that mirrors the moves of the underlying. That makes it easy to go long or short, with leverage, on thousands of markets.
A CFD is an over-the-counter product: your broker is your counterparty, directly or by hedging elsewhere. Hence the importance of a regulated broker. In the EU and the UK, regulators cap leverage, close positions at 50% margin and protect retail clients against negative balances. CFDs are not available to retail traders in the US.
CFD costs: spread, sometimes commission, and swap on positions held overnight. Most retail clients lose money trading leveraged CFDs.