What is Triggers?

Triggers are procedures written in PL/SQL, Java, or C that run (fire) implicitly whenever a table or view is modified or when some user actions or database system actions occur. Triggers supplement the standard capabilities of Oracle to provide a highly customized database management system. For example, a trigger can restrict DML operations against a table to those issued during regular business hours. Triggers are similar to stored procedures. A trigger stored in the database can include SQL and PL/SQL or Java statements to run as a unit and can invoke stored procedures. However, procedures and triggers differ in the way that they are
invoked. A procedure is explicitly run by a user, application, or trigger. Triggers are implicitly fired by Oracle when a triggering event occurs, no matter which user is connected or which application is being used.

You can write triggers that fire whenever one of the following operations occurs:
1. DML statements (INSERT, UPDATE, DELETE) on a particular table or view,
issued by any user

2. DDL statements (CREATE or ALTER primarily) issued either by a particular
schema/user or by any schema/user in the database

3. Database events, such as logon/logoff, errors, or startup/shutdown, also issued
either by a particular schema/user or by any schema/user in the database