ACID stands for Atomicity, Consistency, Isolation, and Durability. These are the properties of a transaction. Here is the small description of all of them:
ATOMICITY: The atomicity property identifies that the transaction is atomic. An Atomic transaction is either fully completed, or is not begun at all. If for any reason an error occurs and the transaction is unable to complete all of its steps, the then system is returned to the state it was in before the transaction was started.
CONSISTENCY: A transaction enforces Consistency in the system state by ensuring that at the end of any transaction the system is in a valid state. If the transaction completes successfully, then all changes to the system will have been properly made, and the system will be in a valid state. If any error occurs in a transaction, then any changes already made will be automatically rolled back.
ISOLATION: When a transaction runs in Isolation, it appears to be the only action that the system is carrying out at one time. If there are two transactions that are both performing the same function and are running at the same time, transaction isolation will ensure that each transaction thinks it has exclusive use of the system.
DURABILITY: A transaction is Durable in that once it has been successfully completed, all of the changes it made to the system are permanent. There are safeguards that will prevent the loss of information, even in the case of system failure.
For more detail Please check the link: http://msdn.microsoft.com/en-us/library/aa480356.aspx