Picking individual winners is hard. So most people don’t — they buy a whole slice of the market at once, through indices and ETFs.
What an index is
A stock index measures the performance of a group of companies. The S&P 500 tracks ~500 of the largest US companies; the FTSE 100 tracks the 100 biggest on the London exchange. An index is just a yardstick — a single number summarising how that basket is doing. You can’t buy an index directly; it’s a measurement, not a product.
Index funds and ETFs
To actually own that basket, you buy a fund that tracks it. An ETF (exchange-traded fund) is the most common: a single security, traded on an exchange like a normal stock, that holds all the companies in the index. Buy one share of an S&P 500 ETF and your money is spread across all ~500 companies at once.
Instant diversification
This is the big draw. Instead of betting on one company, you own a sliver of hundreds:
- One company collapsing barely dents you — the others cushion it.
- You’re not trying to pick winners; you own the whole field.
- It takes one trade, not five hundred.
Spreading risk like this is called diversification, and an index ETF is the simplest way to get it.
Passive vs picking
Buying an index is passive investing — you accept the market’s average return rather than trying to beat it. It’s low-effort and, because these funds barely trade, low-cost. Picking individual stocks (active investing) offers the chance to do better, but most people — professionals included — struggle to beat a simple index over time.
The takeaway
An index measures a basket of companies; an ETF lets you own that basket in a single, low-cost trade. The result is instant diversification — the reason index ETFs are the default starting point for most investors.