In the context of database transactions, ACID is an acronym for Atomic, Consistent, Isolation, and Durable. Transactions provide a simple model of success or failure. A transaction either commits (i.e. all its actions happen), or it aborts (i.e. all its actions are undone). This all-or-nothing quality makes for a simple programming model. The attributes of an ACID transaction are:
A transaction allows for the grouping of one or more changes to tables and rows in the database to form an atomic or indivisible operation. That is, either all of the changes occur or none of them do. If for any reason the transaction cannot be completed, everything this transaction changed can be restored to the state it was in prior to the start of the transaction via a rollback operation.
Transactions always operate on a consistent view of the data and when they end always leave the data in a consistent state. Data may be said to be consistent as long as it conforms to a set of invariants, such as no two rows in the customer table have the same customer id and all orders have an associated customer row. While a transaction executes these invariants may be violated, but no other transaction will be allowed to see these inconsistencies, and all such inconsistencies will have been eliminated by the time the transaction ends.
To a given transaction, it should appear as though it is running all by itself on the database. The effects of concurrently running transactions are invisible to this transaction, and the effects of this transaction are invisible to others until the transaction is committed.
Once a transaction is committed, its effects are guaranteed to persist even in the event of subsequent system failures. Until the transaction commits, not only are any changes made by that transaction not durable, but are guaranteed not to persist in the face of a system failure, as crash recovery will rollback their effects.
The simplicity of ACID transactions is especially important in a distributed database environment where the transactions are being made simultaneously.