Oracle Community Wiki: "Triggers Tutorial"

Read this article, which illustrates constraints and triggers supported by Oracle. A trigger consists of code that is executed when a predefined event occurs at the database, schema, view, or table level. Constraints are simpler than triggers, and are defined at the column or table level, and only check for data integrity.

Click http://www.toadworld.com/platforms/oracle/w/wiki/2203.tutorial link to open resource.