Foot refers to the lower part of the leg and also a measurement for distance, height and length. It is in its singular form.
A. "The wound on his foot has been infected."
B. "I am five foot tall."
Feet is obviously the plural form for foot. It can be utilized when you're referring to the lower parts of both your legs.
"She has been on her feet all day long."
It also stands as a standard measurement for distance, height and length.
"The pit had been dug to reach close to a hundred feet."