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What Is an Index Fund? A Beginner’s Guide
An index fund is an investment fund designed to approximately track a specific market index. Here is how index funds work, what they can cost, and the risks beginners should understand.
IN PLAIN ENGLISH
The Short Answer
An index fund is an investment fund designed to approximately track the performance of a specific market index rather than relying on a manager to continually choose securities in an attempt to outperform it.
An index fund can be structured as a mutual fund or an exchange-traded fund (ETF). Depending on the index it follows, it may hold stocks, bonds, or other securities.
SIMPLE MODEL
Market index → index fund → portfolio of securities
HOW IT WORKS
How an Index Fund Tracks a Market Index
An index fund starts with a benchmark. The fund is then built to approximately follow that index by holding its securities or a representative sample of them.
THE INDEX
The Benchmark Defines What the Fund Follows
A market index represents a defined group of securities. Its methodology determines which investments are included and how they are weighted.
THE FUND
The Fund Builds a Portfolio Around That Benchmark
The fund may hold most or all of the securities in the index, or it may use a representative sample designed to produce similar overall performance.
IMPORTANT DETAIL
The Fund Can Change When the Index Changes
Index providers periodically review their benchmarks. Securities may be added, removed, or reweighted according to the index methodology.
When those changes occur, an index fund may adjust its holdings so that the portfolio continues to approximately follow the benchmark.
KEY PRINCIPLE
The goal of an index fund is generally to approximately track its benchmark, not to have a manager continually select securities in an attempt to outperform it.
SIMPLE EXAMPLE
What an Index Fund Might Hold
A simple example can show the difference between following a broad market index and investing in only a small number of individual securities.
BROAD INDEX
Index Benchmark
INDEX FUND
Fund Portfolio
HOW THEY CONNECT
The Fund Follows the Benchmark
If an index represents 500 large companies, an index fund following that benchmark may hold all 500 securities or use a representative sample designed to produce similar overall performance.
WHAT THIS SHOWS
The index defines the market exposure, while the fund is the investment product built to follow it. The fund does not need to hold every security in exactly the same way to pursue that tracking objective.
This is a simplified educational illustration. Actual index funds may use different portfolio-construction methods, incur expenses, and produce returns that differ from their benchmarks.
WHAT THIS MEANS FOR YOU
What to Look at Before Choosing an Index Fund
The label “index fund” does not tell you everything you need to know. Two index funds can follow different benchmarks, charge different fees, and expose you to very different risks.
CHECK THE INDEX
Understand What the Fund Actually Tracks
Look at the benchmark, the securities it includes, and how those securities are weighted. A broad-market index and a narrow sector index can produce very different portfolios.
REVIEW THE COSTS
Compare More Than the Fund Name
Review the expense ratio and any other costs that may apply. Lower costs can leave more of the fund’s return with the investor, but not every index fund is automatically inexpensive.
LOOK AT THE RISK
Pay Attention to Concentration and Market Exposure
An index fund can still be concentrated in one sector, country, investment style, or asset class. Diversification can reduce some risks, but it does not eliminate the possibility of losses.
FIT THE ROLE
Consider How the Fund Fits Into Your Broader Plan
The usefulness of any fund depends on factors such as your goals, time horizon, risk tolerance, costs, taxes, and broader financial circumstances. The same fund may serve different roles in different portfolios.
BEFORE YOU INVEST
Start With the Fund’s Prospectus
The prospectus can help you review the fund’s objective, benchmark, strategy, risks, and expenses before making an investment decision.
PRACTICAL RULE
Do not choose an index fund based on the label alone. Understand the index, the costs, the risks, and the role the fund would play in your broader investment plan.
COMMON MISUNDERSTANDINGS
What People Often Get Wrong About Index Funds
Index funds are straightforward in concept, but several common assumptions can make their risks, costs, and role in a portfolio easy to misunderstand.
MISUNDERSTANDING
“All Index Funds Are Basically the Same”
Index funds can follow very different benchmarks. One may track a broad stock market, while another may focus on a single sector, country, investment style, or bond category.
BETTER WAY TO THINK ABOUT IT
Start with the underlying index. The benchmark determines much of the fund’s market exposure and risk profile.
MISUNDERSTANDING
“Index Funds Cannot Lose Money”
An index fund can decline when the securities in the index decline. Passive management does not remove market risk or protect an investor from losses.
BETTER WAY TO THINK ABOUT IT
The fund generally follows the market exposure defined by its benchmark, including both gains and losses.
MISUNDERSTANDING
“Every Index Fund Is Well Diversified”
Some index funds hold many securities across a broad market, while others are concentrated in a relatively narrow segment. The number and type of holdings matter.
BETTER WAY TO THINK ABOUT IT
Diversification depends on the fund’s actual holdings and how they fit with the rest of a portfolio. It can reduce some risks, but it does not eliminate risk.
MISUNDERSTANDING
“An Index Fund Should Match Its Index Exactly”
Fund returns can differ from benchmark returns because of operating expenses, trading costs, portfolio sampling, cash holdings, and other implementation factors.
BETTER WAY TO THINK ABOUT IT
The objective is generally approximate tracking, not perfectly identical performance.
REMEMBER
“Index fund” describes an investment approach, not a guarantee of low cost, broad diversification, or positive returns.
FREQUENTLY ASKED QUESTIONS
Common Questions About Index Funds
These are some of the most common questions beginners have after learning how index funds work.
Is an index fund the same as an ETF?
No. An index fund describes an investment strategy that follows a market index. An ETF describes a fund structure whose shares generally trade on an exchange. Many ETFs follow indexes, but ETFs can also use active strategies.
Can an index fund lose money?
Yes. An index fund can decline when the securities it owns decline. Passive management does not remove market risk, and investors can lose money.
Are all index funds diversified?
No. Some index funds track broad markets and hold many securities, while others focus on a narrow sector, country, theme, or investment style. Diversification depends on the fund's actual holdings and does not eliminate investment risk.
Do all index funds have low fees?
No. Many index funds have relatively low operating expenses, but costs vary. Review the expense ratio and any other transaction, account, or fund-related costs that may apply.
Why can an index fund perform differently from its index?
Fund performance can differ from the benchmark because of operating expenses, trading costs, portfolio sampling, cash holdings, and other implementation factors. This difference is commonly discussed as tracking error.
BEFORE YOU INVEST
Review the fund's benchmark, holdings, expense ratio, risks, and prospectus rather than relying on the “index fund” label alone.
SOURCES & METHODOLOGY
How We Verified This Answer
Verestly prioritizes primary regulators and official investor-education resources when verifying investment definitions, fund mechanics, fees, diversification, and risk.
LAST REVIEWED
September 2026
This answer is periodically reviewed for clarity, accuracy, source quality, investment-safety language, and relevant changes.
