The short answer: There is no difference. The long answer: In the old days a mattress foundation was a wooden box that also contained springs or a metal grid. The purpose of this was so that the foundation would have give in order to support innerspring mattresses. These foundations were called box springs. The industry moved away from this in the 1980’s however mattress foundations today are commonly referred to as box springs. We call it a foundation but you can call it a box spring if you like. Tomāto, tomato.