Read an ETF prospectus, particularly the risk factors, on one of the big ETF provider sites. I think iShares (
www.iShares.com) has one of the easiest to navigate sites.
Not all ETFs are equal. Some have uniquie tax aspects because of the assets they hold. If you deicde you want to buy one, read the prospectus for that particular fund very carefully. There is also something called a Statement of Additional Information the ETF must file that has some additional information about the provider's ETFs (or a group of them) generally and goes more in depth about how they invest.
There are two main types of ETFs, tracking ETFs, which try to track a particular stock or bond index, and actively managed ETFs, which have a particular investment objective or thesis. Make sure you know which one you are looking at and whether it is right for you.
Good luck.