There are many ETFs you can trade on the stock markets like airline or oil company stocks and then there are ETFs that are based on size - large cap, mid cap, small cap and you can trade ETFs on that.
One group of ETFs I want to warn you about: Currency ETFs
When you're looking at a stock, normally you want that stock to continue to head higher. You want the stock's company's earning to grow. Revenue growth means that your stock will get healthier.
Currency ETFs relates, in general, to a country's currency.
Imagine that you have a $20 bill and it buys you lunch. But the currency rises and now, you can buy 10 lunches with the $20 because now the currency rose. This may seem like a good thing, but it changes the evaluation of business, economy and Currency ETFs.
It's actually better if the currency value does not fluctuate because if it rises too high or sinks too low, the end result could be be terrible for the Currency ETFs you've invested in. Hyper-inflation or Hyper-deflation could happen as a result.
It's better to look for investments that are made for growth rather than something so unstable as Currency ETFs.