What is the difference between these two "instruments" ICBIT charts
The number after the BTC/USD is the settlement date of that particular futures contract.

You can find a list of all ICBIT futures in this page, and clicking on one will show you all the details, for example: BTC/USD-12.13

As for why the prices are different, it's somewhat outside the scope of a Bitcoin discussion. I really suggest you read up on Futures Contracts. Make sure to fully understand the nature of Futures trading before investing in it.