Zap is a nostr event where you tip someone via LN, the zap event represents a paid lightning invoice, whereas a LN tip is generic, just sending someone a tip directly over LN.
Discussion
I see, but in practice they are both lightening payments, so no real difference really. Please let me know in case my understanding is not correct.
They both involve LN payments but there are different privacy implications. Zaps can be seen by the nostr client and therefore other users can see the sender and recipient whereas regular LN payments are more private/anonymous.