What Is a Stock Market Index? S&P 500, TSX, and Nasdaq Explained

You’ve heard about the S&P 500 rising or falling on the news. But what exactly is a stock market index — and why does it matter for your investments? This guide explains everything you need to know.

What Is a Stock Market Index?

A stock market index is a list of selected stocks grouped together to represent a portion of the market. It’s used to track the overall performance of a market, sector, or investment style. When you hear “the market was up 1.2% today,” they usually mean a major index like the S&P 500.

Indexes themselves aren’t directly investable — but index funds and ETFs track them, allowing you to own a slice of all the companies in an index.

The Major Indexes You Need to Know

How Indexes Are Weighted

Most major indexes are “market-cap weighted” — meaning larger companies have more influence on the index’s performance. Apple, Microsoft, and Nvidia together make up roughly 20% of the S&P 500. This means when big tech does well, the S&P 500 does well — even if hundreds of smaller companies are struggling.

Why Indexes Matter for Your Investments

When you invest in an index ETF like VTI or VOO, you’re not picking individual stocks — you’re buying a tiny piece of every company in the index. This gives you instant diversification, and research consistently shows that most professional fund managers fail to beat their benchmark index over the long term.

• S&P 500 historical average return: ~10% per year before inflation (since 1926)
• TSX Composite historical average return: ~7–8% per year
• 80%+ of actively managed funds underperform their benchmark index over 15 years (SPIVA data)

How to Invest in an Index

You invest in an index by buying an index ETF or index mutual fund through a brokerage account:

1. Open a TFSA/RRSP (Canada) or Roth IRA/401k (US)
2. Fund the account with your desired amount
3. Search for your chosen ETF (e.g., VTI, VOO, XEQT)
4. Purchase — you now own a piece of hundreds of companies