You and I are on the same stair. I go down some stairs and say 저 referring to myself. I'm showing my respect to you by making myself humble. In this case you're my boss or teacher/professor(socially respected occupation in general), senior colleague, or just stranger. 나 is just generally used as "I". Saying 나 can be impolite if you're talking with those people above. There's no equivalent to "you" in Korean. You can say 너 to your close friends with your same age or younger than you, or to kids. When talking to some of those people above, we call them their official title or 선생님, 교수님, 선배님. 그 쪽 is used between strangers or people who first meet each other.

it's almost same.... but "저" is more polite expression... so when i tell someone who is older one or my boss/senior.... i use "저" instead of "나"