Explain the concept of a transaction and its main characteristics in databases.

A transaction in databases is a sequence of operations that is executed as a single unit. The main characteristics of transactions are described by the ACID principle:

• Atomicity: all operations of a transaction are either fully completed or not executed at all.
• Consistency: a transaction brings the database from one valid state to another.
• Isolation: concurrent transactions do not affect each other.
• Durability: after a transaction is committed, its changes are saved even in case of failures.

Transactions are used to ensure data reliability and integrity, especially during complex modifications involving multiple tables or records.