Cryptocurrency-based synthetic assets aim to be attached to that asset without having an underlying asset. That means another entity with the same value as another entity. For example, in traditional finance, synthetic assets are derivatives that simulate the underlying asset, options, futures, stocks, bonds.
Synthetix, on the other hand, is a derivative liquidity protocol that enables the trade of synthetic crypto assets on the Ethereum network. Each synthetic crypto asset (Synth) is an ERC-20 Token that tracks the price of another underlying asset. For example, each sUSD Token tracks the price of 1 US dollar. Using synthetic assets, it is possible to stay in the crypto ecosystem and have the equivalent of the simulated assets. Synthetix, on the other hand, allows trading with these synthetic crypto assets on DeFi.
All Synth, synthetic crypto assets, are supported by SNX Tokens. Synths are generated when SNX Token holders use their SNX Tokens as collateral and interact with Synthetix contracts. Synt's rate of collateral is 750%, but this rate is open to change with community decisions. Synths must be burned when SNX Tokens are wanted to be withdrawn.