— Special Form: while condition forms...

while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil.

There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see Nonlocal Exits).

The value of a while form is always nil.

          (setq num 0)
               ⇒ 0
          (while (< num 4)
            (princ (format "Iteration %d." num))
            (setq num (1+ num)))
               -| Iteration 0.
               -| Iteration 1.
               -| Iteration 2.
               -| Iteration 3.
               ⇒ nil

If you would like to execute something on each iteration before the end-test, put it together with the end-test in a progn as the first argument of while, as shown here:

          (while (progn
                   (forward-line 1)
                   (not (looking-at "^$"))))

This moves forward one line and continues moving by lines until it reaches an empty. It is unusual in that the while has no body, just the end test (which also does the real work of moving point).