Exchange-traded fund (ETF) is a kind of securities traded on the exchange, which represents the ownership of a package of shares and can be sold continuously and redeemed by a package of shares. It provides investors with two trading methods: exchange trading, subscription and redemption. On the one hand, like closed-end funds, investors can buy and sell ETFs on exchanges, and they can sell short and trade margins like stocks (if the market allows these two forms of stock trading); On the other hand, like open-end funds, investors can purchase and redeem ETFs, but when purchasing and redeeming, ETFs exchange fund shares and a basket of stocks with investors (ordinary open-end funds exchange fund shares and cash when purchasing and redeeming). ETFs generally have a lower limit on the number of times of purchase and redemption, and transactions that fail to reach the lower limit can only be conducted through exchanges. In addition, ETFs usually adopt a completely passive management mode, aiming at fitting an index.