Do you have older wood double-hung windows that tend to stick? There are several reasons for this, many of them involving scary, expensive, structural aspects, such as frames bowing inward (best solution: full removal and rebuilding).
Other times, the solution is much easier. If you have a white candle--yes, an ordinary candle that you use to light up a room--your windows will start to move smoother with this easy trick:
Lubricate the window channels by rubbing them down with a white candle. Then open and close the window several times.
The friction will help to spread the candle wax, further lubricating the window tracks. Because the candle wax is white, it will not be noticeable (provided that your window trim and framing is white, too!).
Do not use any kind of oil or WD-40 to lubricate your wood window channels, as this will only attract dust and dirt.