The official definition of the set of properties governing database transactions is mentioned as Atomic, Consistent, Isolated, and Durable (known as ACID). These are the four tenants of a database transaction. Broken down, they mean the following:
Atomic: All work that must be performed by the database inside a transaction – that is, reading data, deleting data, updating data, and/or inserting data – is treated as a single unit of work. It's literally all-or-nothing. If anything fails, then it all fails, and you revert back to the way the universe was before you'd gone and mucked it all up.
Consistent – This is basically The Principle of Least Astonishment; that is, there are no surprises, and everything is as you expect it to be. Foreign keys and other constraints are respected, the state of the data prior to the transaction is used within the transaction, etc. In short a transaction should never, ever leave you with the thought, Huh, how did that happen?!