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.

 

Can I use the command AT in Windows 8.1?

No you cannot. AT command has been deprecated(used no more).
Instead you should use schtasks.exe.

How do you use it?

Open up a command prompt window (AKA DOS window) preferably with administrator privileges.

Just type as shown and you should get the help menu.

C:\Windows\system32>schtasks /?
SCHTASKS /parameter [arguments]
Description:
    Enables an administrator to create, delete, query, change, run and
    end scheduled tasks on a local or remote system.
Parameter List:
    /Create         Creates a new scheduled task.
    /Delete         Deletes the scheduled task(s).
    /Query          Displays all scheduled tasks.
    /Change         Changes the properties of scheduled task.
    /Run            Runs the scheduled task on demand.
    /End            Stops the currently running scheduled task.
    /ShowSid        Shows the security identifier corresponding to a scheduled t
ask name.
    /?              Displays this help message.
Examples:
    SCHTASKS
    SCHTASKS /?
    SCHTASKS /Run /?
    SCHTASKS /End /?
    SCHTASKS /Create /?
    SCHTASKS /Delete /?
    SCHTASKS /Query  /?