Bhai.
p<=>q or p<->q both are same thing , they are notations brother like p=>q or p->q
i said naa p<=>q means p iff q .
p<=>q is logically equivalent is technically wrong statement , here we are talking about the equivalence of p and q. take any to logic statements in this world , any two.
taken ?
good , now make them operators for EX-NOR operation if the outcome is T then your both taken logic formulas are equivalent otherwise not .