Semantics

leg
Он быстро сунул ноги в брюки и побежал. He quickly shoved his legs into his pants and set off at a run.
foot
После лета ноги Пети не влезали в старые туфли. At the beginning of fall, Petya's feet no longer fit into his old shoes.
Конь нетерпеливо переступал ногами. The horse impatiently shuffled its feet.
Russian differs from English in that the word нога may mean either "leg," (i.e. either of the two lower limbs of a biped, including a human being, or any of the paired limbs of animals) or "foot" (i.e. the part of the leg below the ankle joint). This entry is concerned with both of these uses. Common sense will dictate which of the expressions in this entry can or cannot be used to refer to animals or humans.

For the distinction between the word ступня1 and its synonym нога, see Lexical Relationships: Synonyms.