Usage 1:

     WHILE (exp:boolean) statement ...  

While “exp” is true execute statement; when “exp” is false execute following statements
Usage 2:

     WHILE (exp) DO  
     ...  
     ENDWHILE  

WHILE - While “exp” is true execute statement(s); when “exp” is false transfer control to the first statement following the ENDWHILE statement (requires DO [or THEN] after the expression)
ENDWHILE - Transfers control to the closest WHILE statement and marks the end of the WHILE loop (END WHILE is a synonym)
See also: If..Then For…Next