There are many different index funds although most of them are listed on the US stock markets. Only a few index funds are based on world wide indices. The other index funds are specialized based on one of the following:

• geographic area, including the funds that track indices based on a particular local stock exchange. For example EWN (dutch stock market)
  
• total market capitalization of the company, some index funds only invest in large companies (large-caps) and other only in small companies (small-caps),
• some ratio indicating the growth potential such as Price/Earnings (PE), the growth rate or total revenue divided by market capitalization minus the debts plus the total amount of cash or simply the dividends paid in the past. The choice is here between growth stocks and fair valued stocks
  
• sector of the economy, such as health care, energy, transport, consumer goods

Often index funds combine two or more of the above characteristics, for example the VBR fund invests in small-value caps in the US. The more a fund specialized the bigger the difference is with the world index and the bigger the risks.

So before selecting an index fund to invest in always check the index contains enough companies for spreading risks and that the largest holding is only a small percentage of the total fund. This is not true for most sector funds and many country based funds.