A trigger is a named program unit that is stored in the database and fired (executed) in response to a specified event. The specified event is associated with either a table, a view, a schema, or the database, and it is one of the following:
A database manipulation (DML) statement (DELETE, INSERT, or UPDATE)
A database definition (DDL) statement (CREATE, ALTER, or DROP)
A database operation (SERVERERROR, LOGON, LOGOFF, STARTUP, or SHUTDOWN)
The trigger is said to be defined on the table, view, schema, or database.
Have a look at these links.
Wiki definition of Database Trigger:
From Oracle Doc...
... Introduction to Triggers:
... Overview of Data Integrity and Triggers