A transaction is an atomic unit of work that must be completed in its entirety.The transaction succeeds if it committed and fails if it is aborted.Transactions have four essential properties:atomicity,consistency,isolation, and durability(known as the ACID properties).
A (Atomicity) -The sequence of operations must be atomic, either all or no operations are performed.
C (Consistency) - When completed, the sequence of operations must leave data in consistent mode. All the defined relations/constraints must me Maintained.
I (Isolation) - A Transaction must be isolated from all other transactions. A transaction sees the data defore the operations are performed , or after all the operations has performed, it can't see the data in between.
D (Durability) - All oprtaions must be permanently placed on the system. Even in the event of system failure , all the operations must be exhibit.