For example when you speak to someone, depending on age difference, sex, relantionship, etc. you can refer to that person in different ways. "Name+kun" is used when refering to boys. (Used a lot to refer to childs but can also be used between adults but not with strangers). "Name+chan" is usually used to refer to girls. (Used a lot to refer to childs but can also be used between adults but not with strangers). "Name+san" is used to address people of same age or older and actually to refer to anyone. "Name+senpai" Is used to adress people older than yoursel but is generally used by students to refer to their higher grade partners.