An index fund is a portfolio of assets with allocation rules that remain fixed regardless of market conditions, commonly for the purpose of approximating the performance of some market index. The index fund does not have to be linked to an existing index. Typically, an index fund is either a mutual fund or an ETF. An index fund tends to have significantly lower fees and expenses than an actively managed fund because of its passive management style. Taxes are also reduced because less trading lowers realized capital gains for the index fund.