The term “sash windows” can be used interchangeably with the term “box sash windows” in the UK and is generally used to describe the same type of window.
Historically there is a small difference as box sash windows tend to be heavier and more stately in nature than their more modern cousins, but both terms are used within the industry to refer to the same type of window.
The modern version of the classic box sash window uses cords, weights and pulleys to counterbalance the “sashes” in the traditional way. The weights are housed in a ‘box’ created by the internal and external liners on each side of the window.