Explain trigger classes i.e. instead of and after trigger?

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INSTEAD OF: Cause the trigger to fire instead of executing the triggering event or action. It prevents unnecessary changes to be made.

Example: Causes the trigger to fire instead of the update (action)

CREATE TRIGGER Employee_update ON Employee INSTEAD OF UPDATE AS { TRIGGER Definition }

AFTER: execute following the triggering action, such as an insert, update, or delete. These triggers are fired a little late in the process.

Example: Causes the trigger to fire instead of the update (action)

CREATE TRIGGER Employee_update ON Employee AFTER UPDATE AS { TRIGGER Definition }
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