What is an ETF? It stands for an Exchange Traded Fund, which means it is a fund that is traded like a stock. So you are only trading one fund, the ETF of your choosing, which in turn holds a bunch of assets. How is that for a technical answer.
ETFs Can Hold Anything?
Yep. An ETF can hold stocks, bonds, commodities, and other assets. Many of them will follow an index so you can buy and sell an index relatively cheap and of course quickly. Follow the link above for the more in depth explanation by Wikipedia.
That should tell you how popular these are. According to YAHOO! Finance, that is the amount that has been taken into ETFs in 2014 making the total $1.92 trillion. Maybe that is an investment you may want to diversify your own portfolio with.