ETFs - Exchange Traded Funds - are usually passive investments with low TER (Total Expense Ratio). SDPR (State Street) and iShares (BlackRock) are two of the main providers internationally. They have been around since 1980's and their main advantage is that they track an Index (e.g. S&P 500, FTSE100, etc). Hence, they constitute a diversified portfolio of investment.
The low costs and the ability to trade on exchange on a daily basis, just like a stock, has made them very popular.