Triggers are the system generated events.
for example,
u have a table emp,in tat u can create a trigger which will 
be activated if anyone performs any action such as insert, 
update, delete on tat table.In tat trigger u can write code 
which copies the updated info into another table.
we  can provide security to the data by mentioning before 
clause in tat trigger by which we can restrict the 
modifications anyone can make to our table.

CREATE TRIGGER DelhiDel ON [Customers]
FOR DELETE
AS
IF (SELECT state FROM deleted) = ‘Delhi’
BEGIN
PRINT ‘Can not remove customers from Delhi’
PRINT ‘Transaction has been canceled’
ROOLBACK
END

There are mainly two type trigger
1)After trigger
2)Insted of trriger

Baisicalt its like a stroeprocedure...

like as storeprocedure
it is userdefind function ther is no parameters 

1)After trigger:

2)Insted of trriger

A trigger is a stored procedure that goes into effect when 
you insert, delete, or update data in a table. You can use 
triggers to perform a number of automatic actions, such as 
cascading changes through related tables, enforcing column 
restrictions, comparing the results of data modifications, 
and maintaining the referential integrity of data across a 
database.

Triggers are the system generated events.
for example,
u have a table emp,in tat u can create a trigger which will 
be activated if anyone performs any action such as insert, 
update, delete on tat table.In tat trigger u can write code 
which copies the updated info into another table.

A trigger is a stored procedure that goes into effect when 
you insert, delete, or update data in a table. You can use 
triggers to perform a number of automatic actions, such as 
cascading changes through related tables, enforcing column 
restrictions, comparing the results of data modifications, 
and maintaining the referential integrity of data across a 
database.