I am confused about the difference between ↔ (biconditional iff) and ≡ (logical equivalence). For instance, p→q can be rewritten as ∼p∨q. Would it be correct to say p→q↔∼p∨q or p→q≡∼p∨q?
Secondly, is ⇔ another symbol for ≡?
Finally, what's the difference between → and ⇒?
In short, $P \leftrightarrow Q$ is statement that could be either true or false. $P \equiv Q$ means that $P \leftrightarrow Q$ is always a true biconditional (so, $P$ and $Q$ have the same truth value no matter what).
So, one could say that $\neg (P \vee Q) \equiv \neg P \wedge \neg Q$ (DeMorgan's) but you typically wouldn't write $\neg (P \vee Q) \leftrightarrow (\neg P \wedge \neg Q)$.
The arrow $\Rightarrow$ usually is slang for "implies" but different people use it differently. The arrow $\Leftrightarrow$ is usually treated the same way as $\leftrightarrow$.
In case you can use a somewhat philosophical explanation: $\leftrightarrow$ is a logical operator within statements, while $\equiv$ serves to state an equivalence between statements and thus may be thought of as meta-logical.
As Randall explained, $P \leftrightarrow Q$ is a statement $-$ one statement and a logical statement. The $\leftrightarrow$ will cause it to be true under certain truth value distributions for $P$ and $Q$. The same applies to $\neg(P \vee Q) \leftrightarrow (\neg P \wedge \neg Q)$.
For $\neg(P \vee Q) \equiv (\neg P \wedge \neg Q)$ however, you compare two truth tables, the one of $\neg(P \vee Q)$ and $(\neg P \wedge \neg Q)$, two distinct statements. If and only if both are true exclusively for the same truth value distributions, the equivalence applies and so the meta-logical statement $\neg(P \vee Q) \equiv (\neg P \wedge \neg Q)$ is true.
Consider this analogy. You wouldn't say the following:
Prove that 3 + 5.
You might instead say this:
Prove that 3 = 5.
Similarly, following doesn't make sense:
Prove that $p \leftrightarrow q$.
Instead one of these would be correct:
Prove that $p \leftrightarrow q$ is always true.
or
Prove that $p \equiv q$.
