Window sashes are typically found in the double-hung window style, in which one sash is positioned above a lower sash. The lower window sash has the capacity to slide vertically until it is nearly parallel with the upper sash.
However, any type of window which moves (such as the casement window) can also be said to have sashes.
If the window sash breaks, it is possible to purchase a sash replacement kit. This avoids the cost and mess of total window replacement. Also, because it is designed for homeowners to be able to use, it helps avoid labor costs associated with window or sash replacement.