what is the difference b2in phase current and line current?

now i am reading 3 phase transformer but to understand it properly i need the besic concept. please tell me.
I found some other difficulties to read star-delta,star-star, delta star and delta delta connections.
In the books the relationship between the Primary phase current & primary line current,secondary phase current & secondary line current, sec. ph voltage & sec line voltage is not described in the books.zwhere and how do these relationships come?