PrependTo[s, elem] prepends elem to the value of s, and resets s to the result.
PrependTo[s, elem] is equivalent to s = Prepend[s, elem].
PrependTo[s, elem] does not evaluate s.
You can use PrependTo repeatedly to build up a list.
See The Mathematica Book: Section 2.4.4.
See also: AppendTo.