ETFs, exchange–traded funds, are often defined as mutual funds you can trade like traditional stocks and bonds. That difference however is a big one. ETFs allow active traders to ride the daily market movements maximizing profits across every index as well as a wide array of asset classes. Mutual funds offer one daily price. That's it.
If you are looking to diversify, there are hundreds of ETFs out there with low expense ratios. But, like all investments, ETFs are not without controversy. Be sure to do your research before investing.