A square foot vs a foot square

A square foot vs a foot square
 

a square foot vs a foot square

Somone told me the difference but I can't remember and Didn't understand it very well.
Can anyone explain it again?

Un "square foot" es la unidad de medida ("one square foot", "two square feet", etc.) y un "foot square" es un cuadrado que mide un pie por lado.

Ten square feet = one square foot x ten.
Whereas ten feet square = ten feet x ten feet.

To repeat what others have said: (lots of examples are always good)

4 square feet could be a rectangle that is 1' x 4' or 2' x 2', for a total of 4 sq. feet. (The apostrophe means feet)

4 foot square would be a square measuring 4 feet on each side, for a total of 16 sq. feet.