Inside Microsoft SQL Server 2005: T-SQL Programming:
Transactions allow you to define a unit of activity that will be considered atomic—all or nothing. The data will be considered consistent at the beginning of the transaction and at the end. Locks are obtained to isolate data resources, preventing (blocking) other processes from performing incompatible activities against those resources. You can control the degree of isolation of your transaction by specifying an isolation level. I will describe isolation levels in more detail later in the chapter. The database’s transaction log guarantees that a committed transaction recorded within it is durable—that is, the change will reach the database. The aforementioned aspects of transactions are known as ACID(Atomicity, Consistency, Isolation, and Durability).

http://www.webopedia.com/TERM/A/ACID.html:

ACID is the acronym used to describe the four properties of an enterprise-level transaction :

• ATOMICITY: a transaction should be done or undone completely. In the event of a failure, all operations and procedures should be undone, and all data should rollback to its previous state.

• CONSISTENCY: a transaction should transform a system from one consistent state to another consistent state.

• ISOLATION: each transaction should happen independently of other transactions occurring at the same time.

• DURABILITY: Completed transactions should remain permanent, even during system failure.