Investors invest in index funds for various reasons. They invest in index funds to create wealth, achieve financial stability and independence, plan for retirement, vacation, or education. Because they are usually passively managed, meaning fund managers are not actively researching, picking and selling securities, they incur less cost to investors. A single index fund can include all the stocks or securities of an index (e.g. S&P 500) or a good portion of it. 

You can buy index funds from a broker (Fidelity, E-trade, TD Ameritrade, etc.) or from the fund itself. Investors buy index funds at the fund's net asset value (NAV). The NAV is nothing more than the amount of money you need to buy one share of the fund. Here are the steps you need to take to buy index funds: 1) open a brokerage account; 2) fund your account; 3) decide how many shares of the index fund you want to buy; 4) choose your order type. Although index funds are less risky investments compared to stocks, you should always read the fund's prospectus. This is a document that outlines the fund's investment strategy, fees, risks, among other things. Here's a nice little guide on how to evaluate and select index funds for your portfolio.

As an investor, owning index funds comes with risks and benefits. If the index fund's price or net asset value (NAV) goes down, you will lose money and vice versa. There is no certainty that companies whose stocks the index fund holds will prosper or do well. To a large extent, an index fund's NAV is dependent upon the basket of securities (stocks, bonds, cash, etc) it holds. Many factors affect an index fund's share price (NAV), including but not limited to quarterly earning reports of individual companies, regulatory, political, economic and other events.

Investors make money in two major ways. First, they earn money through capital gains. A capital gain is essentially the increase in price of a fund to a price higher than the initial purchase price. Second, investors make money through dividends. Dividends are basically free money you get for simply owning the fund or being a fund holder. Dividends are usually paid to shareholders quarterly. Not all companies pay dividends. Index funds are meant to be kept for the long-term. Here's a nice overview of how investors earn money from mutual funds. Check out this nice comparison between stocks, ETFs and index funds.