Exchange-traded funds, known as ETFs, is an investment fund which offers shares on the stock-market. Generally, ETFs specialize in indexes such as the FTSE 100.
The tragedy of the commons is where shared resources are over-exploited because each individual is following their own self-interest. If each individual only took a limited quantity, there would be enough for everyone. However, due to the self-interest of the individual, many over-exploit the resource and leave nothing left for anyone – hence the tragedy.