What Is Triggers ?

TRIGGERS :

Triggers are one of the database objects, which performs their own operations when user performs any DML operations on specific table.
or

Triggers i a type of stored procedure that implicitly executed when user performs DML operation on the table. it will not accept any parameters.

In simple way triggers fire after DML operations (INSERT,DELETE,UPDATE) it will Executed its own operation which mentioned in trigger.

Like if we insert  new records it will insert into main table and in trigger with new record

Ex: CREATE TRIGGER T1 ON EMP FOR INSERT
      AS
      BEGIN
      INSERT INTO EMP1 SELECT *FROM INSERTED
      END
It will insert the new record into emp and newly inserted record into EMP1(Trigger) like this we can know
newly inserted records in table.
Same with INSERT,DELETE,UPDATE..

TYPES OF TRIGGERS 
AFTER TRIGGER
INSTEAD OF TRIGGER

After Triggers :These are the triggers, which performs their own operation after performing insert, delete, and update operations on a specific table.
syntax :
CREATE TRIGGER TRIGGERNAME ON TABLE NAME
FOR/AFTER{INSERT/UPDATE/DELETE}
AS
BEGIN 
SQL STATEMENT
END