Physical ETFs attempt to track their target indexes by holding all, or a representative sample, of the underlying securities that make up the index. For example, if you invest in an S&P 500 ETF, you own each of the 500 securities represented in the S&P 500 Index, or some subset of them. Physical replication is reasonably straightforward and transparent. Nearly all ETF products in the United States are physical ETFs.
Instead of physically holding each of the securities in its index, a synthetic ETF relies on derivatives such as swaps to execute its investment strategy.
Because they don’t physically hold the securities in which they invest, synthetic ETFs can provide a competitive offering for investors seeking to invest in harder-to-access markets, less liquid benchmarks, or other difficult-to-implement strategies that would otherwise be very costly and difficult for physical ETFs to track.