Explain the concept trigger in sql code example

Example 1: how use trigger in sql

CREATE TRIGGER Product_Details_tr 
BEFORE INSERT ON Product_Details 
FOR EACH ROW 
SET NEW.User_ID = CURRENT_USER();

Example 2: what is a trigger in sql

-- Azure SQL Database Syntax   
-- Trigger on an INSERT, UPDATE, or DELETE statement to a table or view (DML Trigger)  
  
CREATE [ OR ALTER ] TRIGGER [ schema_name . ]trigger_name   
ON { table | view }   
 [ WITH <dml_trigger_option> [ ,...n ] ]   
{ FOR | AFTER | INSTEAD OF }   
{ [ INSERT ] [ , ] [ UPDATE ] [ , ] [ DELETE ] }   
  AS { sql_statement  [ ; ] [ ,...n ] [ ; ] > }  
  
<dml_trigger_option> ::=   
        [ EXECUTE AS Clause ]