As you might imagine, foot odor is usually caused by sweaty feet. But it's a little more complicated than that.
Foot odor is different from other types of body odor. Most body odor emanates from the genitals and armpits, which contain apocrine sweat glands. The rest of the body, including the feet, is covered with eccrine sweat glands. What's the difference? All sweat is basically odorless, but the sweat from apocrine glands contains protein and amino acids that feed bacteria living on the skin. The bacteria generate body odor.