What's the difference between an index fund, an ETF, and a mutual fund?

These three labels describe two different things, so they overlap. "Mutual fund" and "ETF" are wrappers — legal structures that hold a basket of securities. "Index fund" describes strategy: a fund that simply tracks a benchmark like the S&P 500 instead of trying to beat it. An index fund can come in either wrapper. The practical differences: mutual funds price once a day at the closing NAV and you buy in dollar amounts directly from the fund company; ETFs trade like stocks all day at market prices through a brokerage. ETFs are usually more tax-efficient in taxable accounts and have no investment minimum beyond one share. For most long-term investors the wrapper matters less than picking a low-cost, broadly diversified index fund and holding it. See wealthserene.com/tools/portfolio-builder to assemble one.