Option chains list the available option contracts that can be traded on an underlying stock. Not all stocks will have option contracts available to trade and the contracts will vary based on the underlying stock price. The available strike prices available for investors to trade follows a set of standardized rules. They are:

If underlying stock price is less than $25 then strike price intervals are $2.50 ($15, $17.50, $20, $22.50, etc.).
If underlying stock price is between $25 and $200, then strike price intervals are $5 ($75, $80, $85, $90, etc.).
If underlying stock price is more than $200, then strike price intervals are $10 ($310, $320, $330, $340,etc.).

The typical option chain will list the bid and ask prices of each option contract as well volume and open interest. Option contracts are sorted into groups by the expiration month and strike price.

The example below represents a Mastercard (MA) December 2009 options chain.

Call options are listed on the left side and put options on the right side. The blue column down the middle represents the strike price and expiration month.

Looking at the rows in an option chain provides important information on each individual option contract. For the Mastercard December 250 call option contracts, it shows the current bid/ask spread on the contract is $1.40 to $1.45. The Net Change column notes the daily change in the price of the option contract. In this example, the December 250 call option contract has gained +0.50 in value during this trading session. The volume on the 250 call option is 1,179. This means 1,179 contracts have been traded between investors during the trading session. The Open Interest (Open Int) is sitting at 4,184 which means 4,184 December 250 option contracts were already traded coming into the current session.