An Index Fund is a bunch of key stocks listed on a particular stock exchange which captures or replicate the movement of the overall market as a whole rather than a particular stock. Usually it is available in form of a Mutual Fund or a exchange traded fund.
Most of the times Index Funds in form of Mutual Funds are a bunch of stock in a particular segment of the market viz Infrastructure, health, Blue chips etc.
There are a number of Index Funds which follow the growth or movement of the over-all market. Index Fund is considered to be one of the oldest, safest and low cost investment method in the securities market. An Index Fund can be bought through a Fund Manager – Kotak Security, HDFC Mutual Fund, LIC, ICICI Prudential, Reliance Mutual Fund, and many more.
A person investing in Mutual Fund obtains equitable rights to the assets of the mutual fund along with the units. To Understand the functioning of Mutual Funds or an Index Fund we must understand few key terms:
NAV: Net Asset Value is what gives us the value for the each unit of the fund on a day-to-day basis. It actually defines the worth of the MF (each unit). It would be easily calculated -
{Market Value of all Investments in the Fund – (Liabilities + Expenses) } / Outstanding No. of Units in the Fund

So in plain world, NAV on a particular day, gives us the What value of a unit of Mutual Fund we will get if we liquidate the fund on that day.
Income comes from the Mutual Funds in form of Dividend which is exempted from income tax. Since the Index Funds are managed by expert fund managers the risk profile is low though being related to securities market the risk is always there and varies with different funds.
If we see the growth chart of Sensex in last 15 years its commendable and investing the the Index Fund seems to be a wise decision for a small investor.
Though Index Fund is a kind of Mutual Fund but both are not same. An index fund is quit a passive investment where the fund manager mere try to replicate the index which it is following and hence there is very less buying and selling. While in Mutual Fund fund manager keep buying selling different securities keeping short/medium/long term objectives. Cost of managing an index fund is quite lower than mutual fund due to its passive nature.
Image Source: Economic Times