A transaction is an atomic unit of work that either succeeds or fails. Transactions are a key feature of any database system and are what allow a database to implement the ACID properties: atomicity, consistency, isolation, and durability. Altogether, the ACID properties mean that the database must be able to handle units of work on its whole (atomicity), store data in a permanent way (durability), without inter-mixed changes to the data (consistency), and in a way that concurrent actions are executed as if they were alone (isolation).
You can think of a transaction as a bunch of related statements that, in the end, will either all succeed or all fail. Transactions are everywhere in the database, and you have already used them even if you did not realize it: function calls, single statements, and so on are executed in a transaction block. In other words, every action you issue against the database is executed within a transaction, even if you did not ask for it...
