Database related terms

Transaction

A database transaction is a unit of work which the DBMS guarantees to reliably perform independent of other transactions:

reliability: The changes of the transaction are either fully performed or not at all. Unexpected events (e.g. power failure) are guaranteed not to leave the database in a partially changed state.
independence: The actions of the transaction are isolated from other actions which might be concurrently (at the same time) performed by other programs.

A database transaction must be ACID.

When writing code, a transaction is bracketed, i.e. the code has a form:

BEGIN (to denote START TRANSACTION)
... (the code of the action, typically multiple statements)
COMMIT (to denote the end of the transaction and force its guaranteed storage in the database)

In case of an error within the ... code the ROLLBACK operation returns the database to the state before begin.

ACID

Transaction properties:

Atomic: A transaction must be completed or have no effect.
Consistent: A transaction must lead to a state in which all database constraints are met.
Isolated: A transaction must not affect other transaction.
Durable: A transaction changes must get written to persistent storage.

Other terms

In-memory database: fast, data is kept permanently in computer memory; no need to access a disk to respond to queries.
Cloud database: the data and the database system exists “somewhere” online and is fully web controlled.
Distributed database: the data and DBMS utilise multiple computers.