SQL Server Transactions are used to ensure that a series of statements written to modify data will pass the ACID test. They enforce data integrity, accuracy, concurrency, and recoverability of the data in the database. The ACID properties of a transaction are…

• Atomicity – Ensures that a series of statements in a transaction either succeed or fail together.
• Consistent – Enforced by using transaction logs. Ensures that only valid data has been committed will be written to the database.
• Isolated – This is enforced by using locks to control concurrency. Specifically, to make sure that a record being updated by one statement does not interfere with a separate statement trying to modify that same record or records.
• Durable – Once again transactions logs are used as a way of recovering data in case of a database failure. The recovery model of your database may also affect durability.

To learn more, check out this post on transactions and errors or this post on transaction isolation levels.