At First - This event gets triggered for the first iteration
of the loop. Generally used for printing report headers.

At last - This event gets triggered after the last iteration
of the loop. Generally used to print totals.

These come under Control Break Statements :

At FIRST :
     This is used for system Field Heading in ABAP program
At NEW   :  
     This is used to display the fields.
Ex At New Carrid.
       write :  /Carrid.
   endat.
  when ever the new carrid is reached, carrid wiil be 
written.

  
At END   : 
     This is used for row-wise Calculation i.e, Sub-total.
 Ex : At End Carrid.
         uline.
      Endat.

    The end of carrid is reached, a line will be drawn.


At LAST  : 
     This is used for Calculation of Grand total 

 Ex:  At Last.
         wirte : / Grand-Total.
      Endat.
     When entire processing of entire internal table is 
over.