THE ACID TEST v7.0
Despite years of programming and DBA experience, many SQL Server 
administrators have no formal training with SQL Server transactional 
syntax. As a result, these self-taught admins will occasionally be 
tripped up by the subtleties of SQL syntax. A quick review of the so-
called SQL Server ACID test will serve such SQL users well.

According to MSDN Books Online, a transaction is "a sequence of 
operations performed as a single logical unit of work." A logical unit 
of work must exhibit four properties, called the ACID properties, to 
qualify as a transaction: Atomicity, Consistency, Isolation, and 
Durability.
http://click.techrepublic.com/Click?q=ff-sVHVQmsSmyioiwPIxlfpoV6uvdRR

With this in mind, try to imagine a simple transaction like a transfer 
of $123.45 from your savings account to your checking account. This 
small transaction will consist of all four of these properties and will 
involve some sort of database activity.

* Atomicity - All data modifications must be completed as one unit of 
work. You do not want the debit side of the above transaction occurring 
in your savings account without the credit side also occurring in your 
checking account. A transaction that fails this test could potentially 
rob you of $123.45. Example: BEGIN, COMMIT, and ROLLBACK TRANSACTION.

* Consistency - When the above transaction is completed, it must leave 
the data in a consistent state. In other words, structures, linked 
lists, indexes, and datatypes must remain consistent throughout the 
unit of work. You transferred $123.45 from savings to checking; not 
$123.45 from savings and $123.449 to checking due to some rounding 
issue. Example: Uupdating of index pages and datatypes.

* Isolation - Modifications made by concurrent transactions must be 
isolated from the modifications made by any other concurrent 
transactions. Simply putting a second transfer from the savings account 
to the checking account cannot occur until the first one is completed. 
The second transaction cannot see the intermediate state of the first 
transaction. Example: SQL Locking mechanisms

* Durability - The modifications are permanent within the system, even 
in the event of system failure. Example: wWriting of the data to the 
database pages and to the transaction log.

------------------------------------------