What is a trigger in SQL Server?

A trigger is an event or an action that precipitates other events. However a trigger in SQL Server is a special kind of stored procedure that automatically executes when an event occurs in the database server. 

If you want to know what is a stored procedure, follow this link:
http://hodentekhelp.blogspot.com/2011/06/what-is-stored-procedure.html

There are different kinds of events that can occur in a database server such as the following:

• Data Manipulation language events such as,

Insert

Update

Delete

statements on a Table or View.

In these cases if a trigger is defined, it will fire (respond to the event) regardless of any changes are made to the rows of a table.

• Data Definition Language events such as,

Create

Alter

Drop

and certain stored procedures that perform DDL-like operations

• Log-on trigger

These fire when a session is established by a user.