An index is a measure; a fund is an investment vehicle

An index tracks a defined group or rule-based basket. An index fund is a portfolio intended to follow an index; it may hold all constituents or a representative sample. An ETF is a fund structure whose shares trade in a market while that market is open. An ETF can be passive or actively managed and can be narrowly concentrated, so “ETF” does not mean “broadly diversified.”

This lesson uses U.S. registered-fund education from the SEC. It does not describe every exchange-traded product, foreign fund regime, crypto vehicle or individual fund prospectus.

Read exposure, NAV and trading price separately

  • Holdings/exposure: the securities, cash and other positions the fund actually owns, subject to its mandate and disclosures.
  • Index exposure: the benchmark a fund seeks to track or use as a reference; the index itself is not directly purchased through the fund.
  • NAV: calculated per-share net asset value for the fund at its valuation point.
  • ETF market price: the price at which ETF shares trade; it can be above or below NAV and can move with supply, demand and market conditions.
  • Costs and tracking difference: fees and expenses reduce investor results; portfolio choices, cash, rebalancing, timing and other factors can make fund returns differ from an index.

Fictional example: benchmark, NAV and price

Teaching case: a hypothetical index rises 8% over a year. A fund designed to follow it returns 7.2% over the same fictional period after its assumed expenses and other tracking differences. The gap is 0.8 percentage points; the example does not assign the whole gap to one cause or represent a real fund.

On one hypothetical trading day, the fund's calculated NAV is $50.00 while its market price is $49.80. The share trades $0.20 below that NAV, a 0.4% discount relative to $50. Neither number guarantees what a buyer or seller could execute at another time. Check the fund's prospectus, holdings, benchmark definition, costs, valuation time and trading data.

Investor.gov explains that registered mutual funds and ETFs can be passive or active; passive funds may hold all or representative index securities. Its 2025 bulletin compares NAV-based mutual-fund transactions with ETF market trading and discusses costs. It also excludes some exchange-traded products outside the Investment Company Act scope.

A five-part fund check

  1. Identify the legal fund and prospectus.
  2. Read the objective and benchmark; do not assume the label means a broad market.
  3. Inspect actual holdings, concentration and any sampling method.
  4. Find expense and other fees; fees can apply whether performance is positive or negative.
  5. Compare fund return with the same benchmark, currency, time window and calculation basis.

Sources: Investor.gov Updated ETF Bulletin (23 February 2023) and Characteristics of Mutual Funds and ETFs (29 April 2025), both reviewed 26 October 2026. The 2025 bulletin covers registered open-end funds and UITs, not every exchange-traded product or a specific prospectus.