ETFs are part of a group known as index funds.With so much uncertainty in the world, it's hard to reliably predict what a single company — or even a single industry — will do over the long term.To combat uncertainty, the popular advice is to diversify; to hold investments across many asset types, industries, and countries For example, they might invest in the ASX200 — the 200 biggest companies on the ASX.Buying a fund like this is like voting for the share market to go up, rather than any single company or industry, since the fund buys and sells companies as they enter or leave the index