A database transaction is a collection of database operations which are executed all for process user requests for retrieving data from database or updating the database.Database transaction is independent to the other transaction performing in the database.A database transaction follows ACID properties.So A transaction must be Atomic,consistent,isolate, and durable.
A transaction starts with first executing SQL statement as tasks of database transaction. And a transaction ends when it is committed or rolled back.
The properties of database transactions are follows:

Atomicity :Whole or nothing
All the step of a database transaction must be completed or If it is not complete due to any reasons, the database transaction must be aborted.
Consistency :Integrity
The database must be in a consistent before and after perform the task of the database transaction.It means that a database
transaction must not break the database integrity.
Isolation: Data that are using to execution of one database transaction must not be used by another database transaction until the
execution of one database is completed.
Durability:Updation of the database in the transaction will be permanent even if a system failure occurs after the completion of transaction.