What is an ETF? Understanding index funds in 10 minutes
An ETF automatically tracks a stock market index for an annual cost below 0.40%. But between listed TER and real cost (Tracking Difference), physical vs synthetic replication, and fund overlap — this guide covers what most articles don't.
What is an ETF?
An ETF (Exchange Traded Fund) is an investment fund you buy and sell in real time on a stock exchange, exactly like a share. Its defining feature: it mechanically tracks a benchmark index without trying to beat the market.
Buying 1 share of IWDA (iShares MSCI World, price: ~€85 on 25/04/2026) means investing simultaneously in 1,434 companies across 23 developed countries, Apple, Microsoft, LVMH, Nestlé, Toyota and 1,429 others, for an annual cost of just 0.20%.
ETF vs individual stock
If Apple drops 40% on the market, an Apple share loses 40%. If Apple drops 40% within IWDA, the ETF loses roughly 0.5% (Apple represents ~5% of the MSCI World index). That is the instant diversification an ETF provides.
Physical vs synthetic replication
Two methods allow an ETF to track its index:
- Physical replication: the ETF directly buys the stocks in the index (fully or via optimised sampling). Examples: IWDA, VWCE, CSPX. Advantage: full transparency on holdings.
- Synthetic replication: the ETF holds a basket of stocks (often European) and enters a performance swap with a bank that agrees to deliver the index return. Examples: some Amundi ETFs (CW8, SP5C). Main advantage: enables eligibility for the French PEA tax wrapper even for non-European indices.
For dividends: Acc (Accumulating) ETFs reinvest dividends automatically. Dist (Distributing) ETFs pay them out quarterly or semi-annually. For long-term growth, Acc is usually more tax-efficient.
TER vs Tracking Difference: the real cost of an ETF
The TER (Total Expense Ratio) is the annual fee listed in the ETF's KID document, but it is not the actual cost you pay. What really matters is the Tracking Difference (TD): the gap between the ETF's return and its benchmark index return over a year.
The TD can be better than the TER thanks to securities lending: the issuer lends the portfolio's shares to short sellers, earns a lending fee, and passes some of this revenue back into the fund.
Concrete example: IWDA (iShares MSCI World)
→ IWDA underperforms its index by only 0.10% per year in practice, despite a TER of 0.20%. Source: iShares annual reports, 2022–2024 data.
To find an ETF's TD: compare its annualised performance to its benchmark index over the past 3 years on JustETF or the issuer's website (Performance vs benchmark section). A negative TD means the ETF outperforms its index net of fees.
The French PEA tax wrapper: quantified advantage
The French PEA (Plan d'Épargne en Actions) is the most tax-efficient wrapper for ETF investing in France. After 5 years, your gains are taxed at only 17.2% (social contributions), versus 30% flat tax in a standard brokerage account (CTO).
| Scenario | PEA (after 5 yrs) | CTO |
|---|---|---|
| €10,000 invested → +100% in 15 years | €1,720 in tax (17.2%) | €3,000 in tax (30%) |
| €50,000 invested → +150% in 20 years | €12,900 in tax | €22,500 in tax |
| Tax rate applied to gains | 17.2% (social only) | 30% (flat tax) |
How can a global MSCI World ETF be PEA-eligible (which is restricted to European equities)? Via synthetic replication: the ETF physically holds 75%+ European stocks (satisfying the PEA rule), and obtains global performance through a swap contract with a bank. CW8 and WPEA both use this mechanism.
ETFs vs active funds: the data
The SPIVA report (S&P Dow Jones Indices) measures each year the proportion of active funds that underperform their benchmark. European market results (2024):
- → 86% of active global equity funds underperform the MSCI World over 10 years
- → 92% underperform over 15 years
- → Average TER of European active fund: 1.4% / year vs 0.07–0.50% for an ETF
On €10,000 at +8% gross annual return: after 20 years, a 0.20% TER ETF gives €45,300. The same fund at 1.4% TER gives €36,100. Difference: €9,200 lost in fees.
The overlap trap: why duplication matters
The most common beginner mistake: buying multiple ETFs thinking you're diversifying, while they all hold the same companies. Concrete examples:
IWDA + VWCE
MSCI World inside ACWI. You're just adding emerging markets exposure (~18%).
IWDA + CSPX
S&P 500 represents ~68% of MSCI World. Strong US overweight.
IWDA + EQQQ
NASDAQ-100 is tech-heavy. Partial diversification, concentrated risk.
Calculate the exact overlap in your portfolio with our simulator: enter two tickers to get the overlap percentage and common holdings.
Calculate overlapHow to choose your first ETF
1. Define your target exposure
For a first ETF, most long-term investors choose an MSCI World (23 developed countries) or MSCI ACWI (developed + emerging) ETF. Avoid thematic ETFs (AI, water, robotics) as a first holding, too concentrated.
2. Check TER AND Tracking Difference
TER alone isn't enough. Check the TD on JustETF: a 0.20% TER ETF with TD 0.10% is cheaper than a 0.15% TER ETF with TD 0.20%.
3. Check AUM for liquidity
AUM > €500M: very liquid, bid-ask spread < 0.05%. Between €100–500M: acceptable. Below €100M: risk of closure or wide spread.
4. Acc or Dist?
For long-term compounding, Acc ETFs are more efficient: dividends reinvest automatically with no tax drag or manual action required.
5. Check overlaps before adding a second ETF
Use our simulator before combining two ETFs. An overlap > 70% means you're not actually diversifying.
Frequently asked questions
What happens if the ETF issuer (iShares, Vanguard…) goes bankrupt?▾
UCITS ETF assets are legally segregated from the issuer's balance sheet. If iShares goes bankrupt, the shares held in IWDA remain the property of unit holders, they cannot be seized by iShares' creditors. The UCITS directive has mandated this protection since 2009. In practice, an independent custodian (typically a major bank) holds the assets. A liquidator would simply transfer the fund to another manager.
Can I lose all my capital with an MSCI World ETF?▾
An MSCI World ETF cannot go to zero unless 1,434 companies across 23 countries all go bankrupt simultaneously: a theoretically impossible scenario. However, it can lose 40-50% during a major crisis (-41% in 2008, -34% in March 2020). These drawdowns have always recovered within 3-5 years. The recommended minimum investment horizon is 8 years.
What is the difference between MSCI World and MSCI ACWI?▾
MSCI World covers 23 developed markets (1,434 companies). MSCI ACWI (All Country World Index) adds 24 emerging markets (China, India, Brazil…) for approximately 2,800 companies total, representing ~85% of global market cap. Over 20 years, returns are similar (emerging markets have underperformed developed since 2010). ACWI ETFs are slightly more expensive in TER.
Acc or Dist ETF for long-term investing?▾
Acc ETFs are generally preferable for long-term wealth building: dividends compound automatically without tax drag or manual reinvestment. Dist ETFs suit investors who need a regular income stream, or who hold ETFs in a tax-exempt wrapper where dividends are already sheltered.
Summary
An ETF is the simplest and most cost-efficient tool for long-term investing: real costs below 0.20% / year (TD), instant diversification across 1,400+ companies, and strong UCITS regulatory protection. For French investors, the PEA tax advantage (17.2% vs 30%) is a significant net-return multiplier over 15–20 years.
Start with a global MSCI World ETF, check Tracking Difference rather than TER alone, and use our simulator before adding a second ETF to avoid creating unnecessary duplicates.
Check your ETF overlap
Enter two tickers to instantly see their overlap.
e.g. VWCE, CSPX, IWDA, EIMI…
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