Index funds are like mutual fund schemes wherein the fund managers mirror the holding based on the underlying benchmark. However, you can buy these funds only at their net asset value (NAV) determined at the end of the day. In comparison, exchange-traded funds (ETFs) are listed on the stock exchanges. Therefore, you can buy or sell these at any point during trading hours.
ETFs have a higher expense ratio due to trading costs like brokerage and Securities Transaction Tax (STT). Because index mutual funds are not traded and are passively managed, the expense ratio is comparatively lower than ETFs.