Right, and they each have their own meaning. "Introduce your code" could be a command to enter a code to enter a secure sight, or maybe log onto a computer system. Is "introduz" able to be used as an imperative for such a command? I think that in Spanish one could either use the imperative here or the infinitive.
Teu is second person singlar. It's "your" code. And you could never have any doubt.
Seu is third person singular, but often used refering to second person as well (because of "você", which means you, but uses third person conjugations). So, in this case, seu means the same thing as teu = your. But you could have doubts, could be a third person's code.