Question about P ⇒ Q and (¬P ∨ Q)

This week is all about negation. I have one question about this week’s stuff. Why P ⇒ Q as same as (¬P ∨ Q)? I try to analyze this problem. Firstly P implies Q shows all the elements in P must in Q. P is the subset of Q. But I cannot understand why (¬P ∨ Q) means the same thing. Also I am not very sure about the symbol “∨”. Does (¬P ∨ Q) means the union of not P and Q? I cannot see the equivalence between (¬P ∨ Q) and P ⇒ Q.

p.s. I am sorry about posting this blog so late. I wrote the draft copy on last week but I forgot to post it .