ACID is the four main rules that relational database management systems (RDBMS) must provide. For more detailed information about relational databases and SQL Server, you should read “About SQL Server and Comparison with Other Relational Database Management Systems“
It adopts either the All or Nothing principle. If a transaction fails in a transaction, all transactions must fail in any case (server shutdown, failure).
IIf a violation of the defined rules(constraints, cascades, triggers) occurs, all data in the database must remain as it was before the transaction started.
Multiple transactions cannot be changed on the same data at the same time. If a transaction is processing a dataset, it must be isolated from other transactions until the first transaction has finished processing. There are many Isolation Levels in SQL Server. SQL Server uses Read Committed Isolation Level by default. You can read the following articles about Isolation Levels.
“Isolation Levels 1“,
“Isolation Levels 2“,
“Isolation Levels 3”
If a transaction has been successfully committed, this data must be guaranteed even if the system fails. For example, you committed an update, and the server shut down. SQL Server provides this feature with the help of Transaction Log file. You may want to read the article “What is SQL Server Transaction Log“.